Carbon and nitrogen stocks in physical fractions of a subtropical Acrisol as influenced by long-term no-till cropping systems and N fertilisation

The long-term soil management effects on C and N stocks of soil physical fractions are still poorly understood for South American subtropical soils. This study aimed ( i) to evaluate the influence of cereal- and legume- based cropping systems and N fertilisation on C and N stocks of the sand-, silt- and clay- size fractions of a no- tilled subtropical Acrisol in southern Brazil, (ii) to compute the Carbon Management Index ( CMI) for those cropping systems using physical fractionation data, and (iii) to investigate the possible existence of finite capacity of those soil physical fractions to store C and N. Soil samples of a long-term experiment were collected from the 0-2.5 and 2.5-7.5 cm layers of three no-till cropping systems [fallow bare soil, oat/ maize (O/M) and pigeon pea+ maize (P+M)] under two N fertilisation levels (0 and 180 kg N ha(-1)). However, for fallow bare soil, only the non-fertilised sub-plot was sampled. An adjacent native grassland soil was sampled as a reference. The C and N stocks of the three soil physical fractions were higher in the legume- based cropping system (P+M) than in O/M and bare soil, because of the higher residue input in the former. The P+M cropping system restored the C and N stocks in sand- and silt- size fractions to the same levels found in grassland soil. Higher C and N stocks in all physical fractions were also obtained with N fertilisation. The C and N stocks and the C: N ratio were most affected by cropping systems in the sand- and least in the clay- size fraction. Particulate organic matter was found in the silt- size fraction, showing this fraction is not only constituted by mineral- associated organic mater, as commonly believed. Taking grassland soil as reference ( CMI = 100), the CMI ranged from 46, in O/M no N, to 517, in P+M with N, pointing to a better soil management in the latter. The clay- size fraction tended to show a finite capacity to store C and N (48.8 g C kg(-1) and 4.9 g N kg(-1) of clay), which was not verified in sand- and silt- size fractions. The adoption of no- tillage and legume- based cropping systems with high residue input are adequate soil management strategies to improve soil quality and make the agricultural production systems more sustainable in subtropical regions.