Sustainability of New Zealand high‐country pastures under contrasting development inputs. 4. Organic matter components

Abstract

Biomass, N, S, P, C, and Ca composition of shoots, litter, roots and coarse organic matter > 1.3 mm diam., and <2 mm soil fraction are given for two trials under 61 different fertiliser and grazing treatments on a Pukaki/Tekapo high‐country soil after 10–15 years. Effects were primarily related to fertiliser, secondarily to stocking rate, and rarely to the grazing method. While there was an increase in soil C and N and a decrease in C/N ratio of the <2 mm soil fraction with increasing fertiliser rate, the relationships were weak. Spring shoot biomass increased with S and P fertiliser rate and decreased with stocking rate. Surface litter (5–33 t ha‐1) was slightly greater under lax grazing, but total soil root and coarse organic matter (6–57 t ha‐1) increased with stocking rate, with an exponential decrease with soil depth. There were only small treatment effects on %N in organic components, %S in organic matter components increased with S fertiliser rates, and %P increased with P fertiliser rates, especially in the litter layer. There were active roots to 40 cm soil depth. The half‐life of leaf litter decomposition of 10 species from April to September ranged from 0.2 to 38 years. The decomposition rate of litter was positively correlated with C/N and Ca litter concentration over the first fortnight after deposition and with %S in litter over the next 150 days. Microbial biomass decreased with fertiliser level. An incubation experiment indicated that nitrification was occurring. Grazing management treatments had greater effects on soil organic components than on inorganic components.