Dynamics and nature of soil organic matter and its constituent fractions as affected by cropping and tillage.

摘要

An understanding of the long-term impact of tillage systems on soil organic matter (SOM) nature and dynamics is essential to define better strategies for land use, which may favour C sequestration and improve soil quality. The main objective of this research was to assess the nature and dynamics of SOM, and those components of SOM considered to be potential sinks for C, specifically the light fraction and humic fractions, as a consequence of tillage.; Soil samples were taken to the 50 cm depth of an 11-yr old experiment involving no- (NT) and conventional-tillage (CT) and corn and tobacco/rye cropping. The &dgr;13C technique was used to quantify the incorporation of corn-derived C (C4-C) and turnover of native C (C3-C) in SOM and its constituents including the free light (FLF), occluded light (OLF), heavy (HF) and humic fractions. The solid-state 13C cross polarization magic angle spinning nuclear magnetic resonance spectroscopic technique was used to assess the impact of tillage on the chemical nature of FLF and OLF.; Eleven years of NT affected primarily the distribution of total organic C and C4-C in the soil, FLF and HF, resulting in higher quantities of these in the upper 5 cm in NT compared to CT, but it did not increase stocks of these in the soil profile. No-tillage marginally (P = 0.1) increased sequestration of C4-C in the OLF relative to CT. The turnover of C3-C decreased in the order FLF > OLF > HF and it was altered by tillage in OLF only, where it was more rapid by 1.5 times in CT than in NT. However, the OLF accounted for only a small proportion of the soil's total C4- and C3-C (7 and 17%, respectively), hence there was no evidence for the existence of a large pool of either C4- or C3-C stabilized in aggregates. Charcoal accounted for between 72 to 75% of the C3-C in FLF in the 0--20 cm depth, and it decreased the turnover of previous C3 residue C in the FLF by 2.5 times.; The decomposition of plant residues through various stages (plant residues → FLF → OLF) was accompanied by loss of O-alkyl C and accumulation of alkyl C in both CT and NT, but the magnitude of these changes was greater in CT. Consequently, the alkyl/O-alkyl C ratio of LF was higher under CT than NT, indicating that tillage promoted formation of more stable C constituents in LF of surface tilled soils relative to NT.; Transfer of C4-C occurred in all humic fractions indicating that humic substances play an important role in accumulating new organic matter added to the soil. However, there were no distinct differences in the proportions of C3-C replaced by C4-C among humics suggesting that there was no unique active fraction corresponding with the concept of C pools with defined turnover characteristics used in models of SOM turnover.