Effects On Soil Anti-erodibility And Soil Organic Matter Frations In Slope Farmlands With Different De-farming Patterns In South Sichuan Province Of China

Water loss and soil erosion is a serious environmental problem in the world, returning farmland to forest (grass) is the essential measure to remedy soil, control water loss and soil erosion, and improve the ecological environment. After de-farming, how to consider and harmonize the ecological, economic and social benefits and choosing feasible de-farming patterns become an exigent problem to settle. By analyzing soil anti-erodibility and soil organic matter fractions, this paper systematically studied the changes of soil nutrients, water conservation function of soil and litter layer, soil anti-erodibility, fractal dimension characteristics and organic matter fractions (including microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN), water-soluble organic carbon (WSOC), labile organic C, light fraction organic carbon (LFC) and light fraction organic nitrogen (LFN), particle organic carbon (POC) and particle organic nitrogen (PON), humus fraction C and N, soil organic C and N mineralization, aggregate organic C and N in the slope farmland (CK) and its 5-year de-farmed Neosinocalamus affinis plantation (NAP), Bambusa pervariabilis x Dendrocalamopsis oldhami plantation (BDP), Alnus crenastogyne+Neosinocalamus affinis plantation (ANP) and abandoned farmland (AFL) in southern Sichuan Province of China. The main results were found as follows:1) De-farming could increase soil nutrients content, microbe counts and enzyme activities. Contents of soil organic matter (SOM), total N, alkaline hydrolysis N, total P, soil bacteria, fungi, actinomycete and total microbe counts, and invertase, phosphatase and urase activities followed the order of NAP>BDP>ANP>AFL>CK, contents of available P and available K were followed the order of CK>NAP>BDP>ANP>AFL, content of total K followed the order of NAP>BDP>ANP=AFL>CK, and these in 0-15 cm soil layer were higher than in 15-30 cm soil layer. Soil nutrients content were well correlated with enzyme activities and microbe counts, and SOM, total N, alkaline hydrolysis N, total P and total K contents were positively correlated with invertase, phosphatase and urase activities, and bacteria, fungi, actinomycete and total microbe counts. This indicates that the de-farming of slope farmland was beneficial for improvement of soil fertility.2) Soil physical properties and water conversation of soil and litter layer were significantly different under all de-farming patterns. Soil total porosity, maximum water holding capacity and capillary moisture water holding capacity in NAP were better than the others. Litter storage, maximum water-holding capacity, and modified interception of litter layer were NAP>BDP>ANP>AFL. During the process of water holding, the water holding capacity and absorption speed in first 2 h were superior to the rest of time. The equation between the water holding capacity of the litter layer and the immerse time is Q=alnt+b, the equation between the water absorption speed of the litter layer and the immerse time is V=ktn. Overall, NAP had better water conservation function than the others.3) After 5 years'de-farming, the mean weight diameter of soil water-stable aggregates, structural granular index, aggregation state, aggregation rate and physical stability and SOM increased, while the soil structure deterioration ratio, index of unstable aggregate, dispersion ratio, erosion coefficient and index of erodibility decreased, as compared to CK. The synthetic principal component score value of soil anti-erodibility was-4.466～3.436 and followed the order of NAP>BDP>ANP>AFL>CK. SOM was significantly correlated with most of the soil anti-erodibility indices, as well as the synthetic principal component score value of soil anti-erodibility. The results suggested that SOM increasing after slope farmland de-farming could be the key for enhancing the soil anti-erodibility, and NAP was better for enhancing the soil anti-erodibility than other de-farming patterns.4) In the de-farmed plantations and abandoned farmland, the content of>0.25 mm soil aggregates and water-stable aggregates were increased significantly, compared with those in the slope farmland. The fractal dimension of soil aggregate structure was 1.377-2.826 and followed the order of NAP<BDP<ANP<AFL<CK, the fractal dimension of soil aggregate structure decreased with the increase of the contents of>0.25 mm soil aggregate and water-stable aggregate. There were close relationships between fractal dimension of soil aggregate structure, soil physical and chemical properties, microbe counts and enzyme activities under de-farming patterns. This indicated that the de-farming of slope farmland was beneficial for increasing the contents of>0.25 mm soil aggregate and water-stable aggregate, and improving the stability of soil structure. The fractal dimension of soil aggregate structure could be used as an ideal index to evaluate soil fertility, and planting Neosinocalamus affinis in the de-farming slope farmland, which was best for soil improvement in the research area.5) After 5 years'de-farming, SOM, clay contents (<0.001 mm), fractal dimension of soil particles, ratio of soil physical clay (<0.001 mm) and physical sand (>0.01 mm) were increased. Soil fractal dimension of soil particles was 2.507～2.598 and followed the order of NAP>BDP>ANP>AFL>CK. The fractal dimension of soil particles were well correlated with soil physical properties, synthetic principal component score value of soil anti-erodibility, nutrients contents, microbe counts and enzyme activity. This indicated that de-farming of slope farmland was beneficial for soil clay content increase, soil fertility and proportion of particle composition improvement, fractal dimension of soil particle could be used as comprehensive quantitative index to evaluate soil fertility and anti-erodibility for slope farmland de-farming.6) MBC and MBN contents under different de-farming patterns was highest in NAP, and they were followed the order of NAP>BDP>ANP>AFL>CK. In all patterns, MBC and NAP in 0-15 cm soil layer were higher than those in 15～30 cm soil layer. MBC contents in NAP, BDP, ANP and AFL were 111.6～147.0%,78.4-107.4%,59.2～83.9%,28.7-51.3% higher respectively than that in CK, MBN contents were 92.1～107.8%,57.6-78.9%, 45.7-62.3%,27.2～43.6% higher respectively than that in CK, and WSOC contents were 128.4～150.6%,79.2-105.9%,59.3～86.0%,35.8-55.5% higher respectively than that in CK, and MBC, MBN and WSOC contents in upper layer of soil were higher than those in the subsoil. This indicated that de-farming is beneficial for the increase of MBC, MBN and WSOC. Soil microbial biomass can sensitively reflect the effect of re-farming vegetation pattern on soil properties.7) Soil labile organic C contents were 0.121-1.238 g-kg"1 under different de-farming patterns. In the same pattern, the content of labile organic C (CL333), middle labile organic C (CL167) and high labile organic C (CL33) in upper layer of soil were higher than those in the subsoil, and followed the order of CL333>CL167>CL33 The content of labile organic C (CL333), middle labile organic C (CL167) and high labile organic C (CL33) and soil carbon management index in different soil layers followed the order of NAP>BDP>ANP>AFL>CK. Soil labile organic C (CL333, CL167and CL33) in 0-15 cm soil layer were all higher than those in 15～30 cm soil layer. Total N, alkaline hydrolysis N, total P, total K, soil bacteria, fungi, actinomycete, invertase, phosphatase and urase activities and the synthetic principal component score value of soil anti-erodibility were significantly correlated with high labile organic C (CL33), middle labile organic C (CL167) and labile organic C (CL333) and soil carbon management index, this indicated that total N, alkaline hydrolysis N, total P, total K, soil bacteria, fungi, actinomycete, invertase, phosphatase and urase activities had close relationships with different soil labile organic C and soil carbon management index. The study on soil labile organic C and soil carbon management index can predict the change of soil fertility in de-farming land and soil anti-erodibility.8) The contents of soil LFC and LFN, heavy fraction organic (HFC) and nitrogen (HFN), the distribution ratios of LFC and LFN, C/N ratio of light fraction and heavy fraction, and the storage of LFC and LFN all showed a tendency of NAP>BDP>ANP>AFL>CK. At the same time, the contents of LFC, HFC, LFN and HFN, the proportion of LFC and LFN in total organic carbon and nitrogen, respectively, and C/N ratio of light fraction and heavy fraction in 0-15 cm soil layer were all higher than those in 15-30 cm soil layer.9) De-farming can increase contents of soil POC and PON, incorporated organic carbon (IOC) and nitrogen (ION), and improve the distribution ratios of soil POC and PON, and the C/N ratio of particle organic matter. These indexes in upper layer of soil are all higher than those in the subsoil. The storage of POC and PON, IOC and ION showed a tendency of NAP>BDP>ANP>AFL>CK. Compared with CK, the storage of POC and PON increased 60.9%～162.4% and 54.5%～136.5% respectively, and IOC and ION increased 54.6%～119.8% and 50.4%～103.4%, respectively. This indicated that de-farming is the key to increase soil POC and PON, IOC and ION.10) The contents of soil humus and labile humus compositions of C and N, extractable humus, and proportions of labile humus compositions of C and N followed the order of NAP>BDP>ANP>AFL>CK. This indicated that NAP is more efficient to increase the soil humus C and N fractions and soil labile humus C and N fractions. The increased rate of soil labile humus C and N under different de-farming patterns were higher than that of soil humus C and N fractions respectively, indicating that soil labile humus C and N fractions were more sensitive to different de-farming patterns than soil humus C and N fractions.11) Soil organic carbon (SOC) mineralization under different de-farming patterns was significantly different in every soil layer, and the cummualtive CO2-C in upper layer of soil is higher than that in the subsoil and follows the order of NAP>BDP>ANP>AFL>CK. In the first five days of the mineralization, the cummualtive CO2-C increased sharply in the first five days and then changed moderately, in the whole culture, the cumulative amount of mineralized SOC were increased based curve of objective function. Compared with AFL, NH+-N, NO3-N and total inorganic N in NAP, BDP, ANP and CK all increased. Soil net nitrogen mineralization rate followed the order of NAP>BDP>ANP>AFL>CK.12) SOC and SON contents of all different size aggregates under different de-farming patterns followed the order of NAP>BDP>ANP>AFL>CK respectively, indicating that SOC and SON in soil aggregates would increase with the vegetation restoration in de-farming land. The contents of SOC and SON in soil aggregates increased with the aggregate size decreased, showing a change tendency of V, that means the SOC and SON contents in small particles and micro-aggregates (>5 mm and<0.25 mm) were high, while in intermediate size aggregates was low, and<0.25 mm aggregate SOC and SON were the highest. De-farming increased the distribution ratios of SOC and SON in the lager particle-sized aggregates, and decreased the distribution ratios of SOC and SON in the lower particle-sized aggregate.