Redistribution of a Pulse Input of 15N in a Sown Perennial Subtropical Grass Pasture With and Without Light Cultivation

Renovation of old· grass pastures by light cultivation to improve the availability of soil N has given unpredictable results. A better understanding of N movement in the soil-plant system is needed to develop management for long-term productivity. Our work traced the movement of a pulse input of labelled N through the soil-grass system with and without annual light cultivation. Labelled N from the mineral N pool peaked first in the microbial biomass, then in roots, tops, litter, and soil organic matter. Annual light cultivation reduced the uptake of labelled N by grass. It initially increased immobilisation of 15N into fine litter and later into soil organic matter. Effects on the soil microbial biomass N were not detectable. The effect of soil moisture on renovation warrants investigation. Light cultivation cannot be recommended yet as a reliable way to renovate grass pastures.