Responses of soil nutrients and microbial activity to the mill-mud application in a compaction-affected sugarcane field

Keywords: Colwell P, compaction removal, deep trench, enzyme activities, labile organic C and N, microbial biomass, mineral N, sugarcane yield.

Compaction, a type of soil disturbance that commonly occurs with over use of heavy machinery in fields, leads to severe crop yield loss and soil health decline worldwide (Shah et al. 2017). Compaction may substantially impact soil functions, such as restricting root growth and limiting soil water and nutrient availability. For example, it has been reported that 20% of sugarcane yield losses were due to compaction in Bundaberg, Australia (Bell et al. 2002), 12–23% wheat grain yield decline due to compaction was reported in a field study conducted in Morocco (Oussible et al. 1992) and an average of 16% sugarbeet yield decline was reported in Hungary by Lipiec et al. (2003). The effect of compaction on crop productivity, soil carbon (C) and nutrient pools and dynamics has been assessed in a range of field studies (George et al. 2011; Colombi and Keller 2019; Shukla et al. 2020). It has been reported that compaction, excessive tillage and the depletion of soil organic matter are the main contributors to sugarcane (Saccharum officinarum) (Garside et al. 2005), maize (Zea mays) (Chen and Weil 2011) and spring barley (Hordeum vulgare) (Czyż 2004) yield decline. However, the response of soil microbial activity to compaction removal practices has been less explored. Such studies could provide some insight into the underlying mechanisms and improve the knowledge of field managers to enhance their decision-making in applying appropriate field management practices to minimise the effects of compaction.

Adequate soil bulk density, organic C and nutrient pools support soil microbial performance and further determine soil health conditions (Magdoff and Weil 2004; Flavel and Murphy 2006). High soil bulk density (>1.47 g cm⁻³ for clay soil and >1.8 g cm⁻³ for sandy soil) affects soil health condition by reducing soil porosity, which decreases soil microbial activity and restricts root growth (Li et al. 2002). Labile C, which is largely produced by organic matter decomposition processes, is considered the main food source of soil microbes (Valenzuela-Solano and Crohn 2006). Labile C limitation or organic matter depletion may result in soil health decline (Liu et al. 2018). While the importance of mineral nutrients for crop growth has been recognised, organic nutrients are also strongly related to crop yield and soil health condition (e.g. Liu et al. 2018). Brackin et al. (2015) highlighted that the organic form of N was more suitable for plant root uptake, while inorganic nitrogen (N) concentration in soil was, to some extent, overestimated for plant nutrition due to its extraction method. Organic phosphorus (P) may not be directly utilised by plants; however, enhancing microbial activities would increase P mineralisation, thus enhancing P availability (Gilbert et al. 2008). In addition to other physical (e.g. soil bulk density) and chemical indicators (e.g. soil nutrient pools and availability), soil microbial biomass C and N, microbial activities and the bacterial-to-fungal ratio are also considered important and sensitive soil health indicators because these parameters can change rapidly in response to shifts in environmental conditions (Bhandari et al. 2018; Bünemann et al. 2018). As a key driver of nutrient cycling, the soil microbial community plays a vital role in soil C mineralisation (Nicolardot et al. 1994). A measure of overall microbial activity was suggested to be an important indicator of the impacts of field management practices on soil health (Schloter et al. 2003; Arias et al. 2005).

In recent decades, sugarcane yield decline has occurred in many sugarcane growing regions in Australia due to compaction and inappropriate field practices. Depletion of soil organic matter and overuse of heavy machinery in the field are reportedly responsible for declines in soil structure and sugarcane yield, particularly in monoculture sugarcane-growing areas (Pankhurst et al. 2003; Garside et al. 2005). To prevent further yield loss and soil health decline, the application of organic amendments such as mill-mud (a primary by-product from sugar production) or compost has been suggested to increase soil organic matter, reduce soil bulk density and improve soil structure (Qureshi et al. 2007; Pattison et al. 2011; Ishak and Brown 2018). Mill-mud application is an efficient way to increase sugarcane yield by improving soil physical, chemical and biological properties (Naidu and Syers 1992; Morris et al. 2007; Gilbert et al. 2008). McCray et al. (2015) reported that broadcast application of mill-mud significantly increased sugarcane yield via boosting soil nutrient concentrations in a Florida sandy soil. Several studies worldwide have indicated that mill-mud application substantially increased soil organic C and nutrient transformation by providing substrates for microbial utilisation (Fauci and Dick 1994; Lima et al. 2009; Tan 2009). It has been suggested that the mill-mud can also modify soil microbial pool size and microbial C use efficiency in a poorly structured soil (Fang et al. 2020). Hence, mill-mud application is considered one of the most suitable field practices to ameliorate or remove compaction impacts. Based on the above discussion, more attention should be paid to the response of microbial activities and changes in soil nutrient pools to organic amendments used for removing compaction. This will enable better predictions of soil health condition in the context of boosting crop yield and developing sustainable agriculture.

This study aimed to investigate and compare the responses of soil microbial activity and nutrient pools to different compaction removal practices, such as deep trenching mill-mud application and shallow furrow application, in sugarcane fields. The following hypotheses were tested: (a) the application of mill-mud would increase soil nutrient pools including total and labile pools, (b) different application methods (shallow furrow application and deep trench application) would lead to different distribution patterns of organic C and nutrients at different depths in the soil profile and (c) mill-mud application and soil compaction stress removal would increase the size of the microbial community and microbial activity, which would further increase soil health condition and sugarcane yield.

The experimental site was located in a sugarcane producing area, Burdekin (19°30′S, 147°20′E), Queensland, Australia. The soil is a Mesonatric Brown Sodosol in Australia (Isbell 2016) which is also classified as a Solonchak based on the FAO world reference base (WRB) for soil resources (IUSS Working Group WRB 2014). The mean annual temperature is 29.2°C and mean annual precipitation is 741.0 mm (Australia Bureau of Meteorology 2021). The experimental field had been cropped with sugarcane for the past 15 years. As a common practice, fine agricultural gypsum was applied to a depth of ∼10 cm using the broadcasting method. Gypsum was applied to improve soil structure and calcium (Ca) concentration (5 t ha⁻¹). Soil at this field site has been affected by compaction due to routine use of heavy machinery (e.g. harvester) for management and harvesting. Mean values of surface soil properties from six composite samples (>30 soil cores) taken prior to the study across the paddock (c. 50 ha) include pH (6.01), EC (29.1 μS cm⁻¹), clay (4.5%), silt (37.3%), magnesium (Mg; 477 mg kg⁻¹), lead (Pb; 7.7 mg kg⁻¹), copper (Cu; 9.5 mg kg⁻¹), nickel (Ni; 4.7 mg kg⁻¹), titanium (Ti; 398 mg kg⁻¹), manganese (369 mg kg⁻¹) and iron (Fe; 5.9 g kg⁻¹) (Supplementary Table S1a, b). Overall variations among these composite samples were relatively small with coefficients of variation <15% for most parameters measured (e.g. pH, EC, particle size, Mg, Pb, Cu, Ni, Ti and Fe) at various soil depths (Table S1a, b). This provided a basis for comparing the field treatments. In this study, there were four treatments: (a) control treatment (CK, without application of mill-mud), (b) mill-mud shallow furrow (c. 20 cm depth) addition treatment (MS, mill-mud shallow furrow), (c) deep trench treatment (DT, deep trench without mill-mud) and (d) deep trench mill-mud application (c. 40 cm depth) treatment (MD, deep trench with mill-mud application). Mill-mud was applied in January 2018 at a rate of 35 t ha⁻¹ (dry weight based) and sugarcane (S. officinarum) harvested in July 2019. Amendments were applied to the targeted subsoil levels (c. 20 and 40 cm depths) in each trench 2 weeks before mung bean (Vigna radiata (L.) Wilczek) planting using a large trencher and conveyer spreader to drop mill-mud into trenched slots in the area where the cane row would be planted. Mill-mud materials (a mixture of mill-mud and mill-ash) were slightly acidic (pH 6.4) and mill-mud nutrients included C (9.1% of dry matter), N (0.42% of dry matter), P (0.73% of dry matter) and potassium (0.61% of dry matter). Soil sampling took place in July 2019 with six replicates from each treatment. Adjacent CK (26.05 ha), MS (26.05 ha), DT (8 ha) and MD (8 ha) treatments were divided into six subplots. Further description of soil sampling is shown in Fig. S1. The soil cores (five) were randomly sampled with a 5-cm diameter auger and divided into four layers of 0–10, 10–20, 20–40 and 40–60 cm depth in each subplot and bulked together to make a composite sample. Fresh soil samples were sieved (<2 mm) and stored at 4°C prior to chemical and biochemical analyses (within 1 week).

Sugarcane yield was measured by harvesting the entire plot of four treatments, and yield was calculated based on weight of harvested cane divided by its entire treatment area (t ha⁻¹). Therefore, unfortunately, there was no replicate of the sugarcane yield result. This has been the common practice in the sugarcane farming due to the short window for harvesting in Burdekin. It was acknowledged that the yield data (without statistical analysis) could only be used as indication of impacts of mill-mud amendments.

The soil sand, silt and clay contents were measured using a Maxing sizer (Malvern Panalytical, UK). Soil bulk density was estimated by soil weight divided by its volume using the method described by Maynard and Curran (2007). Soil pH and EC values were measured in a 1:5 volumetric suspension of soil to distilled water (Rayment and Lyons 2011). Soil mineral N (NH₄⁺-N and NO₃⁻-N) was extracted by 2 M KCl at a 1:5 ratio of soil to extractant using an end-over-end shaker for 1 h, filtered by a Whatman 42 filter paper (Rayment and Lyons 2011) with concentrations of NH₄⁺-N and NO₃⁻-N determined by a SEAL AA3 Continuous Segmented Flow Analyser (SEAL Analytical Limited, USA). Soil Colwell P was extracted by 0.5 M NaHCO₃ at a 1:5 ratio of soil to extractant using an end-over-end shaker for 16 h, filtered by a Whatman 42 filter paper (Rayment and Lyons 2011) with concentrations of Colwell P measured by a SEAL AA3 Continuous Segmented Flow Analyser. Soil total C (TC) and N (TN) contents were measured by the combustion method using a LECO CNS-2000 analyser (LECO Corporation, MI, USA). Soil total P was measured using an inductively coupled plasma optical emission spectrometer after digestion (ICP-OES; Perkin Elmer; Optima 8300). Hot water extractable organic C (HWEOC) and hot water extractable total N (HWETN) were measured using the method described by Chen et al. (2000). Briefly, 4.0 g (oven-dry equivalent) of fresh soil was incubated with 20 mL of water in a capped Falcon tube at 70°C for 18 h. After incubation, the tubes were shaken on an end-over-end shaker for 5 min and filtered through a Whatman 42 filter paper, followed by a 0.45-μm filter membrane. Concentrations of dissolved organic C and total extractable N in the filtrate were determined using a SHIMADZU TOC-VCPH (Shimadzu, Kyoto, Japan) TOCN analyser. All results were expressed on an oven-dry basis.

Soil microbial biomass C (MBC) and N (MBN) contents were measured by fumigation-extraction method using an Ec conversion factor of 2.64 (Vance et al. 1987) and an En conversion factor of 2.22 (Wilson 1988). Concentrations of soluble C and N of the fumigated and nonfumigated soil samples were determined using a SHIMADZU TOC-VCSH/CSN TOCN analyser. Fluorescein diacetate hydrolysis, widely accepted as an accurate and simple method for measuring the overall microbial activity in a range of environmental samples, was used to measure soil microbial activity in this study (e.g. Green et al. 2006). The activities of soil β-glucosidase (hydrolysing cellulose to glucose) at pH 6.0 and soil phosphatase (hydrolysing phosphomonoesters) at pH 6.5 were measured according to methods described by Tabatabai (1994).

Univariate ANOVA was used for soil properties data using the IBM SPSS Statistics 23 software package (IBM Corp., Armonk, NY, USA). Differences at P ≤ 0.05 between treatments were considered significant and all variables were tested for normality of distribution using the Kolmogrov–Smirnov test. Pearson linear correlations were used to describe the relationships between soil properties. In addition, data on all soil properties were analysed using principal component analysis (PCA) to distinguish the effects of mill-mud addition and deep trench application on soil.

Soil contained around 5% clay, 38% silt and 57% sand in topsoil (0–10 cm), and clay content increased with soil depth. Soil bulk density in MS was significantly (P < 0.05) lower than other treatments at 0–10 cm (1.07 g cm⁻³) and 10–20 cm depths (1.38 g cm⁻³), while there were no significant differences in bulk density among CK, DT and MD treatments at these two depths (1.47–1.60 g cm⁻³ at 0–10 cm and 1.54–1.71 g cm⁻³ at 10–20 cm) (Table 1). At 20–40 cm depth, MD bulk density (1.49 g cm⁻³) was significantly lower than MS (1.78 g cm⁻³) (Table 1). The CK had the highest bulk density (2.32 g cm⁻³) among all treatments at 40–60 cm depth, while DT had the lowest bulk density (1.67 g cm⁻³) (Table 1). All soil samples were slightly acidic. Soil pH values in CK (5.72) and MS (5.92) were significantly (P < 0.05) higher than MD (5.09) at 0–10 cm depth (Table 1). A similar trend in soil pH was observed at 10–20 cm depth (Table 1). For the deeper soil profiles (20–60 cm), soil pH in CK and MS was significantly higher than DT and MD (P < 0.05). In addition, the treatments that received mill-mud (MS and MD) inputs had higher pH than treatments without mill-mud (CK and DT) for deeper soil profiles (20–60 cm). Total C and N contents in the MS treatment were generally higher than for other treatments at 0–20 cm depth. Additionally, the MD treatment had significantly higher TC and TN concentrations than other treatments at depth of 20–40 cm. Interestingly, at 40–60 cm depth, DT had the highest TC content (0.18%) and MD had the highest TN content (0.058%) (Table 1).

Table 1.  Selected physicochemical properties of sugarcane field soil under different field mill-mud applications.

There were no significant differences in concentrations of HWEOC among all treatments for surface soil (0–10 cm), despite MS having relatively higher HWEOC concentration (Table 1). For the 10–20 cm depth, the DT and MD treatments had significantly greater HWEOC than MS and CK, with the lowest HWEOC in CK. There were no significant differences in HWEOC between the two deep trench treatments (DT and MD). At 20–40 cm depth, the MD treatment had the highest HWEOC concentration (196.9 mg kg⁻¹, P < 0.05), followed by DT and MS treatments, with the lowest value in CK (Table 1). A similar trend was found for the 40–60 cm depth (Table 1).

There were no significant differences in HWETN concentrations among all treatments for surface soil (0–10 cm). At 10–20 cm depth, HWETN concentrations were significantly greater for the MD than the other three treatments, while there were no significant differences among CK, MS and DT treatments (Table 1). At 20–40 cm depth, as expected, HWETN concentration in the treatments receiving mill-mud (MS and MD) were at least 70% higher (P < 0.05) than the two treatments without mill-mud. At the 40–60 cm depth, mill-mud application significantly increased concentrations of HWETN in MS and MD treatments (3.39 and 2.18 mg kg−1, respectively), compared to CK and DT (1.65 and 0.66 mg kg⁻¹, respectively). Interestingly, surface applied mill-mud did not increase labile C store in the entire soil profile (0–60 cm) while deep trench applied mill-mud significantly increased labile C store in the entire soil profile (Table 1).

Concentrations of soil NH₄⁺-N were in the range of 4.00–4.12 mg kg⁻¹ among MS, DT and MD treatments, being significantly (P < 0.05) higher than in CK (3.77 mg kg⁻¹) (Table 1) for the surface soil (0–10 cm). However, there were no significant differences in NH₄⁺-N concentrations among MS, DT and MD. At the 10–20 cm depth, the MS treatment increased NH₄⁺-N concentrations (4.18 mg kg⁻¹) while CK had the lowest NH₄⁺-N concentrations (3.73 mg kg⁻¹). Concentrations of NH₄⁺-N in MS and MD were significantly higher (P < 0.05) than in the other treatments at 20–40 cm depth. In addition, we found MD had the highest (P < 0.05) NH₄⁺-N concentration (4.10 mg kg⁻¹) at 40–60 cm depth, followed by MS (3.24 mg kg⁻¹). No significant differences in soil NO₃⁻-N concentration were found in the 0–10 cm depth among all treatments. The MS had significantly higher (P < 0.05) NO₃⁻-N concentration (1.10 mg kg⁻¹) than the other treatments at 10–20 cm depth. The treatments receiving mill-mud (MS and MD) had significantly (P < 0.05) higher NO₃⁻-N concentration than the other treatments at 20–40 cm depth. At 40–60 cm depth, MD had the highest NO₃⁻-N concentration (0.95 mg kg⁻¹, P < 0.05), while there were no significant differences among the CK, MS and DT treatments.

Total P concentrations in MS and MD significantly increased (P < 0.05) after mill-mud application at 10–20 cm (380.8 mg kg⁻¹ in MS) and 20–40 cm depths (294.5 mg kg⁻¹ in MD) compared with CK and DT at the same soil depth (Fig. 1a). As expected, MS treatment increased total P concentration at the targeted soil level (10–20 cm) but also increased total P for the surfac soil (0–10 cm) (Fig. 1). The MD treatment had the highest soil total P for the deep layers (20–40 and 40–60 cm) (Fig. 1). However, total P concentration decreased with soil depth (20–60 cm) in the CK treatment (Fig. 1a).

Fig. 1.  Responses of soil total P (a) and Colwell P (b) to different mill-mud application in a compaction-affected sugarcane field (n = 96). Control treatment (CK, without mill-mud), mill-mud shallow furrow (c. 20 cm) addition treatment (MS, mill-mud shallow furrow), deep trench treatment (DT, deep trench without mill-mud) and deep mill-mud application (c. 40 cm) treatment (MD, deep trench with mill-mud application). Columns for treatments with the same letter above are not significantly different at P < 0.05 within each soil depth.

Colwell P concentration decreased with soil depth (20–60 cm) in the treatments without mill-mud application (CK and DT) (Fig. 1b). The mill-mud application treatments (MS and MD) exhibited significantly greater amounts of Colwell P in soil compared with CK and DT. The MS treatment increased Colwell P concentration in the surface soil (0–20 cm) but had no impact on the deeper soil (20–60 cm) (Fig. 1b). However, the MD treatment significantly increased Colwell P levels in deeper soil (20–60 cm) but had no effects on levels in surface soil (0–20 cm) (Fig. 1b).

Mill-mud application significantly (P < 0.05) increased levels of MBC in the MS (0–20 cm) and MD (0–60 cm) treatments, compared with CK and DT treatments (Fig. 2a). Notably, the DT treatment increased the concentration of MBC in the lower soil profile (20–60 cm) compared to CK. Increases in MBC were mainly in the upper layers (0–20 cm) for the MS treatment (Fig. 2a). As expected, increases in MBC were in deeper layers (20–60 cm) for the MD treatment (Fig. 2a). In terms of MBN, the MS treatment increased the concentration of MBN, similar to the pattern for MBC (Fig. 2b). However, the MD treatment had the lowest MBN level in nearly all layers (Fig. 2b). In general, MBC levels decreased with soil depth, but this trend was not clear for MBN. Interestingly, mill-mud application (MS and MD) significantly (P < 0.05) increased the microbial C:N ratio in comparison to CK, although this pattern became unclear in the MS treatment in the 40–60 cm layer (Fig. 2c). Notably, DT treatment had no impact on the microbial C:N ratio in the 0–40 cm soil profile, but significantly increased microbial C:N ratio at 40–60 cm depth (Fig. 2c).

Fig. 2.  Responses of soil microbial biomass C (MBC) (a) and N (MBN) (b) and microbial C:N ratio (c) to different mill-mud application in a compaction-affected sugarcane field (n = 96). Control treatment (CK, without mill-mud), mill-mud shallow furrow (c. 20 cm) addition treatment (MS, mill-mud shallow furrow), deep trench treatment (DT, deep trench without mill-mud) and deep mill-mud application (c. 40 cm) treatment (MD, deep trench with mill-mud application). Columns for treatments with the same letter above are not significantly different at P < 0.05 within each soil depth.

Overall, microbial activity for CK, MS and DT treatments decreased with soil depth, but MD tended to have increased microbial activity down to 40 cm and then declined in the 40–60 cm layer. It is notable that the DT treatment increased microbial activity in the deeper soil profile (20–60 cm) compared to CK, which was confirmed by the MBC result. The MS treatment had the highest microbial activity in the surface soil (0–20 cm) while the MD treatment had the highest MBC at 20–40 cm depth (Fig. 3a).

Fig. 3.  Responses of soil microbial and enzyme activities to different mill-mud application in a compaction-affected sugarcane field (n = 96). Control treatment (CK, without mill-mud), mill-mud shallow furrow (c. 20 cm) addition treatment (MS, mill-mud shallow furrow), deep trench treatment (DT, deep trench without mill-mud) and deep mill-mud application (c. 40 cm) treatment (MD, deep trench with mill-mud application). Columns for treatments with the same letter above are not significantly different at P < 0.05 within each soil depth.

Activities of β-glucosidase and phosphatase enzymes were highest at 10–20 cm depth for the MS treatment, and highest at the 20–40 cm depth for MD, which corresponded well with the target depth of mill-mud application for the MS and MD treatments. In the CK treatment, activities of both enzymes decreased with soil depth, while in the DT treatment, β-glucosidase activity tended to decline with soil depth, but phosphatase increased in the 20–40 and 40–60 cm layers (Fig. 3b, c).

Fig. 4 shows clearly that field management of MS, DT, and MD obviously increased plant cane yield. It is noteworthy that the DT treatment increased plant cane yield (157.4 t ha⁻¹) more than for the MS treatment (152.1 t ha⁻¹). As expected, field management also increased sugar yield; however, there was only a slight increase for the MS treatment while DT and MD treatments boosted sugar yield.

Fig. 4.  Responses of cane yield (a) and sugar yield (b) to different mill-mud application in a compaction-affected sugarcane field. Control treatment (CK, without mill-mud), mill-mud shallow furrow (c. 20 cm) addition treatment (MS, mill-mud shallow furrow), deep trench treatment (DT, deep trench without mill-mud) and deep mill-mud application (c. 40 cm) treatment (MD, deep trench with mill-mud application).

Soil bulk density was significantly negatively correlated with many biological properties such as microbial activity, enzyme activity, MBC and MBN (r = 0.557–0.768, P < 0.01) (Table 2). Soil pH was also negatively correlated with microbial activity, phosphatase activity, MBC and microbial C:N ratio (r = 0.308–0.519, P < 0.05) although there was no significant correlation between pH and β-glucosidase activity (Table 2). In addition, TC, TN and total P were positively correlated with microbial activity, enzyme activities, MBC and MBN (r = 0.285–0.880, P < 0.05). As expected, soil Colwell P was positively correlated with microbial activity, enzyme activities, MBC and MBN (r = 0.505–0.696, P < 0.01). Soil NH₄⁺-N was positively correlated with microbial activity, enzyme activities, MBC and microbial C:N ratio (r = 0.334–0.566, P < 0.05) while NO₃⁻-N only showed a positive correlation with enzyme activities (r = 0.354–0.542, P < 0.05). Notably, soil labile pools also showed positive correlations with microbial activity, enzyme activities, MBC, MBN and microbial C:N ratio (r = 0.364–0.846, P < 0.05), while labile N showed no correlation with microbial C:N ratio (Table 2).

Table 2.  Pearson correlation coefficients (r) among soil physicochemical and biological properties (n = 96).

The PCA results showed that principal components (PCs) PC1 and PC2 explained 57.0% and 12.2% of the data variance, respectively (Fig. 5). Treatments were clearly separated from each other based on PC1 and PC2. At the 0–10 and 10–20 cm depths, the MS treatment was clearly separated from the other three treatments along PC2. The CK, DT and MD treatments could be separated from each other along PC2 at different soil depths (Fig. 5). At the 20–40  and 40–60 cm depths, the MD treatment was separated from the other treatments along PC1, while the DT treatment was separated from the CK and MS along PC2 at these depths (Fig. 5). The parameters with the highest correlation coefficients (>0.85) for PC1 were total C, labile C, MBC and microbial and β-glucosidase activities; and the parameter showing the highest correlation coefficient (>0.85) for PC2 was pH (Table S2a, b).

Fig. 5.  Score plot of principal component analysis (PCA) showing the separation of soil samples collected under different mill-mud application and loading values of the individual soil parameters for PC1 and PC2 for soil samples (n = 96). Control treatment (CK, without mill-mud), mill-mud shallow furrow (c. 20 cm) addition treatment (MS, mill-mud shallow furrow), deep trench treatment (DT, deep trench without mill-mud) and deep mill-mud application (c. 40 cm) treatment (MD, deep trench with mill-mud application).

In the present study, it was assumed that adjacent soils (under different treatments) within a paddock were similar in their origin and parent materials. This assumption has been generally accepted and applied as the basis of many paired-site studies (e.g. Chen et al. 2004). This has been supported by similar results of particle size and elemental analyses among composite samples across the paddock prior to the treatment (Table S1a, b; Materials and methods section). In addition, the paddock was uniform due to the same cropping, ploughing and management history. We acknowledge the limitation of this study, as for many other paired-site studies, conducted using pseudo-replication. Soil compaction and the decrease in soil organic matter are key issues that limit the continuous increase of sugarcane yield (Liu et al. 2018). The compaction is brought about by the overuse of heavy machinery in harvesting while lower organic matter results from long-term monoculture and cultivation. To address sugarcane yield decline due to deterioration of soil health (Garside et al. 2005), mill-mud has been widely applied to the surface soil of sugarcane fields, but this application is limited to within 20 km of the sugar mill due to the freight cost. Mill-mud application to soil can enhance soil health and fertility through an increase in soil organic matter and supply of nutrients (Orndorff et al. 2018), improved soil physical structure and retention of water (Fang et al. 2020), increased soil microbial activity (Ishak and Brown 2018) and enhanced soil pH buffering capacity (Medina et al. 2015).

It has been reported that deep trench application of composted municipal-biosolid to sugarcane fields can help plant growth and increase sugarcane yield (Viator et al. 2002). However, soil nutrient pool and microbial activity responses to compaction removal may vary with different depths of mill-mud application, soil type and environmental conditions such as soil temperature and moisture content. The PCA results showed a clear separation of soil samples under different treatments (Fig. 5), indicating that overall, mill-mud application methods had diverse and significant impacts on soil properties and function. Changes in the key soil parameters (including soil pH, total soil C, labile C, MBC and microbial and β-glucosidase activities) induced by the different treatments contributed to the separation of samples.

Mill-mud application significantly reduced soil bulk density at targeted levels (c. 20 cm soil depth in MS and c. 40 cm soil depth in MD) compared to CK. In addition, DT treatment also reduced bulk density down to the 60 cm depth (Table 1). This indicated that deep trench and mill-mud application could be considered compaction removal methods to reduce soil compaction stress. Early research conducted on a clay soil showed that deep trench field practices would reduce soil bulk density down to 102 cm depth to improve soil water infiltration and cotton root growth (Heilman and Gonzalez 1973). In the present study, deep trenching did not have a significant impact on soil pH while mill-mud application slightly increased soil pH which concurs with results of other studies (Morris et al. 2007; Liu et al. 2018). Notably, increased soil pH via mill-mud application will provide better environmental conditions for soil nutrient turnover which would further increase crop yield (Morris et al. 2007).

Labile pools of soil organic matter are key fractions in terms of soil quality as they are available for microbial consumption and play an important role in soil nutrient cycling (Hu et al. 1997). Chen and Xu (2005) suggested that the hot water extractable soil organic C and total N measured as soil labile C and N pools are some of the best indicators of the impact of field management practices on soil fertility. It has been reported that compost application can increase soil organic matter content (Pankhurst et al. 2002; Rahman et al. 2007; Luo et al. 2010) and provide the bioavailable organic C and N for soil microbial growth and for stimulating soil nutrient recycling (Mendham et al. 2003). In this study, deep trench application of mill-mud significantly increased concentrations of HWEOC and HWETN in the deep soil layer due to the inputs of labile organic matter from mill-mud (Table 1). However, DT treatment alone also increased HWEOC concentrations throughout the soil profile compared with the CK, which might be ascribed to the increased mineralisation of organic matter because of the disturbance effects induced by deep trenching. As expected, mill-mud application substantially increased soil total C and total N concentrations in the present study due to the presence of high C and N in the mill-mud. This result indicates that application of mill-mud increases soil C and N stock and, with the deep trench application method, mill-mud can significantly increase soil total C and N concentrations in deep soils (40–60 cm depth). Eghball et al. (2004) suggested that soil mineral N concentrations increased following organic amendment application to soil. Our study also showed an increase in concentrations of soil NH₄⁺-N and NO₃⁻-N following mill-mud application; however, this occurred at different depths for the MS and MD treatments. The MS treatment slightly increased mineral N in the 10–20 cm layer while the MD treatment slightly increased NH₄⁺-N and NO₃⁻-N in the 20–40 cm layer.

Mill-mud application significantly increased soil P concentration (MS and MD) in the applied layer mainly due to the substantial amount of P in the mill-mud (c. 0.73% P of dry matter). Increased P availability (Cowell P) following mill-mud application could also contribute to (i) enhanced competitive sorption between low molecular-weight aliphatic acid (via mill-mud application) and P for soil sorption sites and (ii) increased metal complexation and dissolution reactions by organic matter, resulting in increased release of sorbed P (Bolan et al. 1994; Hue et al. 1994; Maurice 1995). Additionally, mill-mud application may improve soil moisture conditions (Parmar and Sharma 1996) and cause structural shifting of soil microaggregates which decrease the number of potential P sorption sites (Wang et al. 2001). Importantly, mill-mud application increases soil mineralisable C which boosts microbial incorporation of P, resulting in increased available P (Chen et al. 2000; Guppy et al. 2005). In the present study, we also found that mill-mud application increased soil labile C and available P simultaneously. However, the DT treatment alone had no effect on soil P content.

Mill-mud application to sugarcane fields can supply available organic C and reduce compaction stress for microbial activities. Liu et al. (2018) indicated that organic amendment application might not only increase soil MBC, but also enhance soil MBN content. In the present study, soil MBC contents in the mill-mud applied layers (0–10 cm in MS and 20–40 and 40–60 cm in MD) were significantly higher compared with other treatments. This may be ascribed to the organic C inputs to the targeted and adjacent soil layers in the mill-mud applied treatment, and subsequently more litter, which may increase MBC contents in the soil profile. Lima et al. (2009) confirmed that presence of carbohydrates from organic amendment influenced synthesis of humic substances and biological properties in general. Higher concentrations of MBN in MS at the 0–20 cm depth were observed compared with other treatments, which was similar to the trend in MBC. However, the DT treatment had the highest MBN for deep layers (20–60 cm), which may contribute to leaching and improved porosity due to decreased bulk density. Leaching of soluble organic C and N provides an available energy source for utilisation by soil microbial communities, leading to an increase in soil microbial activity (Mendham et al. 2003). This is supported by the positive correlations between soil microbial biomass content and HWEOC and HWETN contents in the present study. Li et al. (2002) reported that decreased bulk density increased microbial pools as it is negatively correlated with bulk density. This statement supports our finding that the DT treatment increased MBC and MBN content in deep soil layers (20–60 cm), which suggests that compaction removal would increase soil microbial pools. In the present study, we found that MD had the highest MBC concentration of all treatments, indicating that the deep trench method could help mill-mud boosting of soil MBC content.

Field application of soil organic amendments has been reported to significantly increase soil enzyme activity (Albiach et al. 2000). In the present study, overall microbial, β-glucosidase and phosphatase activities tended to be higher in the mill-mud application than the other treatments (10–20 cm in MS and 20–40 cm in MD). This was likely due to the higher organic inputs from the mill-mud as shown by the significant and positive correlations between enzyme activities and soil total C and N as well as soil HWEOC and HWETN contents. Phalke et al. (2016) showed that soil organic C content had a significant correlation with soil β-glucosidase activity after organic amendment of a soybean–maize cropping system. In addition, it has been reported that the increase in soil enzyme activities can be related to decreased soil bulk density (Kaiser and Heinemeyer 1993). In this study, microbial activity showed a significant negative correlation with soil bulk density and thus the improved enzyme activities in the mill-mud and deep-trench applied treatments, suggesting a partial alleviation of constraint due to soil compaction removal practices.

Organic amendment field application can significantly improve crop yield (Goswami et al. 2017). Tejada and Gonzalez (2003) argued that the significantly increased grain yield after organic amendment application could be attributed to increased soil organic matter. Liu et al. (2018) demonstrated a significantly increased cumulative sugarcane yield over 4 years in an organic amendment applied treatment, which linked improved soil chemical properties to the resulting yield. In the present study, increases in plant cane yield and labile C took place in deep trench and mill-mud applied treatments. This indicated that soil labile C from slow decomposition of mill-mud plays a vital role in increasing sugarcane production. A possible explanation could be lignin-like products from decomposition of plant tissue benefiting the formation of soil humus, which further benefits plant growth (Tan 2009), and carbohydrates released from mill-mud being a very active soil organic component and ready energy source for improving microbial activities (Pascual et al. 1999). Ghimire et al. (2017) also suggested that soil labile C pools played important roles in maintaining soil fertility. In addition, soil labile C, which is extracted by hot water, is biologically active and highly related to soil microbial activity (Chen et al. 2004). Mill-mud would also increase soil P and mineral N to further improve crop yield by overcoming P deficit (Balemi and Negisho 2012) and balancing the soil N:P ratio (Dai et al. 2020). In the present study, labile C, mineral N, total P and Colwell P were significantly positively correlated with MBC, MBN, enzyme activities and microbial activity. Therefore, the labile organic C and nutrients derived from the mill-mud decomposition enhanced soil microbial activity and nutrient cycling and ameliorated soil compaction stress for sugarcane growth. This subsequently improved soil chemical and physical conditions and increased the plant cane yield. In the present study, surface applied mill-mud slightly increased cane yields but there was no significant increase in soil labile C stores and microbial activity in comparison to CK. Interestingly, the DT treatment significantly increased plant cane yield and moderately increased soil labile C store and microbial activity. As there was no organic matter input in the DT treatment, the significantly reduced soil bulk density resulting from this practice increased soil aerobic condition (Drew 1983), availability of nutrients (Jusoff 1991) and decomposition of native organic matter which may increase nutrient cycling (De Neve and Hofman 2000), labile C store, microbial activity (Li et al. 2002) and crop yield (Wallace and Terry 1998). However, this field compaction removal practice may not be good for sustainable farming as huge soil organic C losses would occur in a long run without extra organic matter input. This study clearly demonstrated that mill-mud deep trench application would simultaneously increase plant cane yield and maintain soil health by removing soil compaction, increasing soil labile C store and boosting microbial activity.

It was clearly demonstrated that application of mill-mud improved soil physiochemical properties including bulk density, nutrient availability and labile organic C and N (HWEOC and HWETN) contents, thereby stimulating nutrient cycling, and enhancing soil health and crop yield. Deep trench application of mill-mud stratified nutrients supplied the soil microbial community in all soil layers, enhanced nutrient cycling processes and thus increased soil health and sugarcane productivity. In contrast, surface applied mill-mud only increased surface nutrients and organic matter content with a limited crop yield improvement. Surface mill-mud application would also result in C loss from soil. It is noteworthy that the DT treatment unexpectedly increased sugarcane yield, which was mainly attributed to increased mineralisation of native organic matter, increased nutrient availability and subsequent ameliorated soil compaction, improved soil structure and redistribution of soil organic C through the entire soil profile. Further research should focus on microbial community composition shifts and functional genes involved in C and N cycling in response to compaction removal to further explore the mechanisms responsible for soil microorganism regulation of nutrient cycling in sugarcane farming systems.

Supplementary material is available online.

The data that support this study will be shared upon reasonable request to the corresponding author.

The corresponding author, Chengrong Chen, is an Associate Editor of Soil Research, but was blinded from the peer review process for this paper. The other authors declare no conflicts of interest.

This work has been supported by the Cooperative Research Centre for High Performance Soils whose activities are funded by the Australian Government’s Cooperative Research Centre Program. Moreover, it is acknowledged that Xiangyu Liu is financially supported by a Soil CRC Top-up Scholarship and a Griffith University Postgraduate Research Scholarship.