Decomposition and distribution of 15N labelled mustard litter ( Sinapis alba) in physical soil fractions of a cropland with high- and low-yield field areas

Abstract

High-yield (HY) areas of an agricultural cropland were characterized by different positions on a slope and lower silt and clay contents, compared to low-yield (LY) areas, and this was associated with differences in water regime and C and N turnover. To understand differences in N flows of HY and LY areas, a combination of 15N tracer techniques and physical fractionation procedures was applied. Within 570 d after application of 15N labelled mustard litter to an agricultural cropland, the distribution of 15N was measured in particulate organic matter (POM) fractions and in fine mineral fractions (fine silt- and clay-sized fractions). After 570 d, only 2.5% of the initial 15N amount was found in POM fractions, with higher amounts in POM occluded in aggregates than in free POM. After this period, stabilization of the initial 15N in fine silt- and clay-sized fractions amounts to 10% in HY, but 20% in LY soils. 70% to 85% of the added 15N were lost. Initial decomposition of labelled material was faster in HY than in LY areas during the first year, but the remaining 15N amounts in POM fractions of the different areas were similar after 570 d. 15N amounts and concentrations in mineral-associated fractions increased within 160 d after application. From 160 to 570 d, HY and LY areas showed different 15N dynamics, resulting in a decline of 15N amounts in HY, but constant 15N amounts in LY soils. The results indicate faster decomposition processes in HY than in LY areas, due to different soil conditions, such as soil texture and water regime. The higher silt and clay contents of LY areas seem to promote N stabilization in fine mineral fractions. As a whole, N flows were higher in HY compared to LY areas, thus supporting higher yields and accelerated organic matter degradation due to higher N supply.