Factors influencing the soil-plant nitrogen cycle of hill pastures under conventional and organic management

Abstract

Nitrogen (N) is the major nutrient that limits pasture growth in New Zealand. Here we test the hypothesis that N supply to herbage from soil microbial mineralization is a function of both the quantity and quality of the soil organic matter, and that this underlying process is similar under conventional and organic management. Preliminary results for October- November 2002 from nine sites with a wide range of soil N status at AgResearch, Ballantrae show that microbial-mineralized N supply from the soil to herbage was the dominant factor controlling the differences in herbage growth. Herbage N was also highly correlated with the soil N supply, as estimated from a 56-day laboratory incubation of soil (0-7.5 cm, and 7.5-20 cm depths). For these soils, spring herbage production could be estimated from the negative relationship with the C:N ratio of the topsoils. This suggests the over-riding factor in the N supply at the nine sites was the quality of soil organic matter in the topsoils. Quality is enhanced through the growth of legumes that in turn depend on the P status of the soil. The soil parent material at some sites (1996 organic farmlets) is calcareous mudstone, which has a high P status, and may explain some differences in site fertility not explained by past P applications. The relationship between the quantity and quality of organic matter and microbial N mineralization in the four farmlets that had organic management appeared to be on the same trend-lines as those in the conventional farmlets, indicating that the underlying net N mineralization process was similar under conventional and organic management. Other factors statistically related to herbage yield and soil net N mineralization were some groups of nematodes and microbial P, but not microbial biomass C or N. Keywords: N mineralisation, non-chemical farms, organic farms, soil fertility