Effect Of Soil Redistribution On Seasonal Change In Soil Organic Carbon Fractions

Soil organic matter plays a key role in the improvement of soil physical, chemical and biological properties. Soil organic matter can be divided into three main pools:active or labile, slow and passive. However, total SOM is not a good measure of management-induced change in soil quality because the bulk of total SOM is recalcitrant passive pool material. The active pool, which is comprised of microbial biomass and labile organic compounds, makes up less than5%of the soil organic carbon. Soil microbial biomass regulates all SOM transformations and is considered to be the chief component of the active organic matter pool. The slow pool usually makes up20-40%of the total organic C and the recalcitrant pool makes up60-70%of the soil C. The active pool of SOM forms a relatively small portion of total SOM but it plays important roles in maintaining and monitoring soil quality. Soil erosion traditionally conceived as three-step process involving the detachment, transport and deposition of the soil particles. Soil erosion transport light density and fine particle soil materials from hill down to low-lying land areas, which can lead to carbon loss and subsequent sequestration. Soil erosion adversely affects SOC content by directly removal of soil and reduction of the top soil depth; it also impacts plant growth and soil biological activity. The objective of this study was to assess the effect of soil redistribution on seasonal changes in soil nutrients and soil organic carbon fractions. Soil samples were collected from three depths (0-10,10-20and20-30cm) with three replications in April,2011and July, September and December,2012representing spring, summer, autumn and winter seasons from terraced slope in loess plateau, China. The collected soil samples were subjected to analyses for physico-chemical properties such as particle size analysis (soil texture), pH, soil moisture (SM), bulk density (BD), soil organic carbon (SOC), total nitrogen (TN), available nitrogen (AN), available phosphorus (AP), microbial biomass carbon (MBC), permanganate oxidizable organic carbon (POXC). POXC was determined by spectrophotometer, EOC and CFC were determined by fumigation extraction method, MBC was calculated as the difference in C concentration between the fumigated and non fumigated samples. On cultivated sloping land where tillage practices occur, soil redistribution not only depends on erosion/deposition by water, but also influenced by tillage erosion. Soil redistribution (erosion and deposition) patterns were estimated in terraced slope using the fallout137Cs technique. In details, total soil redistribution estimated from Mass Balance Model2indicated that soil eroded from upper slope position by39.74t ha-1yr-1and deposited on lower position by27.70t ha-1yr-1. Soil redistribution by tillage was determined by using the TEP model developed by Lindstrom. Tillage-induced soil redistribution was25.06t ha-1yr-1soil losses from upper slope and4.78t ha-1yr-1soils deposited on the lower slope. Water-induced soil redistribution was calculated by subtracting tillage-induced soil redistribution from total soil redistribution. The estimated water-induced soil redistribution indicated that soil eroded from upper slope position by14.68t ha-1yr-1and22.93t ha-1yr-1gains on lower position. Our results indicated that tillage erosion was much dominant at our terraced slope. At all depths, C/N ratio was higher in lower site than in upper site in all seasons. Our result agrees with other studies, indicating that tillage erosion results in high SOC inventory at depositional sites.At0-30cm, mean of SOC, TN, AN, AP, CFC, MBC and POXC are significantly higher under lower site than under upper site in all seasons. The result showed that average content of AP, CFC, MBC and SMC was highest in spring at0-10cm depth. SOC, TN and POXC were little altered by seasons but not significantly. In both sites at all depth, AN was highest in autumn, due to fertility. In upper site at0-30cm, mean of EOC increased106.18%from spring to winter. In lower site at0-30cm, mean of EOC decreased31.79%from spring to summer then increased51.53%from summer to winter. In general, in upper site at0-10,10-20and0-30cm depths, CFC was no differences between seasons. In upper site at0-30cm, mean of CFC increased14.55%from spring to winter. In lower site at0-30cm, mean of CFC decreased34.12%from spring to summer then increased16.16%from summer to winter. In lower site at0-30cm, mean of CFC was significantly higher in spring than in other three seasons. In upper site at0-30cm, the MBC decreased46.31%from spring to autumn then increased6.71%in winter. In upper site at0-30cm, MBC was highest in spring, lowest in autumn and winter and intermediate in summer. In lower site at0-30cm, the MBC decreased52.14%from spring to autumn then increased9.89%in winter. In lower site at0-30cm, MBC was higher in spring than in other three seasons. In upper site at0-30cm, POXC decreased32.68%from spring to winter. In lower site at0-30cm, the mean of POXC decreased26.46%from spring to autumn then increased24.5%in winter. In general, in both sites at all depths, POXC was no significantly differences between seasons.Many measurements in this study were significantly correlated with each other. Soil redistribution rates because of tillage erosion gave a better correlation with SOC, TN, CFC and MBC. Soil redistribution rates because of water erosion gave a better correlation with AP. The strong and positive relationships between soil137Cs and MBC in both sites suggest that they are probably moving along similar physical pathways.