Effects of modifying plant carbon inputs on nitrogen distribution in intact cores of a perennial grass pasture

Abstract

Perennial pastures can accumulate large quantities of roots and surface litter of high C:N ratio, which may reduce N availability to the plant by stimulating microbial immobilization. We studied the effects of modifying carbon inputs from roots and litter on the distribution of nitrogen (N) in plant and soil fractions of an old N-deficient green panic (Panicum maximum var. trichoglume) pasture. Intact pasture cores were taken from the field to a glasshouse, and the surface litter was removed before applying the following treatments: (i) surface litter added, (ii) roots pruned to kill approximately 60% of roots, and (iii) plant shoots removed. A small pulse of 15N as ammonium sulfate was added to the soil surface, and the cores were destructively sampled on several occasions over the following 4 months. Litter addition had little effect on N uptake by uncut plants. When plant shoots were removed, litter markedly reduced plant N uptake. Litter increased N and 15N in microbial biomass and N and 15N stabilized in non-biomass soil organic matter, and reduced loss of N from the cores. Root pruning had little effect on N distribution, except for an initial reduction in plant uptake. Removal of pasture shoots markedly increased soil nitrate and loss of 15N, and decreased non-biomass organic N and 15N. Recovery of 15N in non-biomass organic matter was around three times greater than 15N in microbial biomass, and was closely associated with microbial CO2 production. There was evidence that 15N entered the non-biomass organic matter by both abiotic and microbially mediated processes. In these pastures, the non-biomass soil organic matter may be a more important sink for N than the microbial biomass.