Effects Of Hedgerow On Purple Soil Water Properties And Its Mechanisms

Purple soil mainly distributes in the Sichuan Basin of China. Purple soil layer is shallow, loose, and extremely prone to soil erosion. Soil erosion in sloping cropland of purple soil is very serious. Hedgerow could control soil erosion, improve soil nutrient status, and regulate farmland microclimate. Hedgerow has good ecological, economic and social benefits. It is an important agronomic and biological measure to prevent soil erosion. Herbal hedgerow grows fast and has dense root system. The plant of herbal hedgerow is simple, and the cost is low, and it is able to conserve soil and protect slope in a relatively short time after the plant. Hence, herbal hedgerow is very suitable to be applied in hill areas for soil and water conservation. In this study, the soil conservation mechanisms of hedgerow were studied from both the underground system and the above-ground system of the hedgerow. The above-ground part was researched by growing Leucaena leucocephala and Vetiveria zizanioides hedgerows in standard runoff plots (10°,15°) and by analyzing soil physical, chemical and microbiological characteristics. The underground part was researched by growing grasses, Medicago sativa, V. zizanioides, Cynodon dactylon and Paspalum natatu. Root characteristics of herbs were analyzed by WinRHIZO root scanning system, and soil physical and chemical characteristics were analyzed, and anti-shear ability and anti-scourability were also measured. The effects of hedgerow on soil erosion, slope micromorphology, surface soil particle composition, nutrients distribution and microbial populations in purple soil sloping cropland were analyzed quantitatively. The main results are as follows:(1) In sloping cropland of10°,15°, both L. leucocephala and V. zizanioides hedgerows showed soil and water conservation effects. According to the data of runoff plots monitored in2010and2011, the two kinds of hedgerows had better effect on sand sediment than on surface runoff. At the same declivity, V. zizanioides hedgerow demonstrated better ability than L. leucocephala hedgerow in controlling soil erosion.1year after the plant of hedgerow, declivity showed more obvious effect on soil erosion than hedges. But this phenomenon changed in the next year. The micromorphology of sloping cropland changed in hedgerow plots2years after the plant, represented as sand deposition before the hedgerows, slope declivity decreased between two hedgerows lines, and ridges formed under hedgerow lines. The hedgerow of V. zizanioides changed the micromorphology more significantly than that of L. leucocephala.(2) In sloping cropland of10°,15°, the mechanical constitution and nutrient distribution of plots changed after the plant of hedgerow. In2010, small soil particle (silt and clay) accumulated at the foot and in the middle of the slope. The nutrient loss on the top of the slope reduced, and nutrients enriched at the foot of the slope, among which the enrichment of soil organic matter were most distinguishing, and the next were total nitrogen, total phosphorus, available phosphorus and available potassium, respectively. Total nitrogen did not enriched before hedgerows. The amount of total potassium increased in all plots. In10°sloping cropland, small soil particles increased more significantly under V. zizanioides hedgerow than that under L. leucocephala hedgerow. The intercept effects of these two kinds of hedgerows to soil particles were sand> silt> clay. L. leucocephala hedgerow affected total nitrogen, available phosphorus and total potassium more significantly than V. zizanioides hedgerow did, while V. zizanioides hedgerow affected soil organic matters, available nitrogen, total phosphorus and available potassium more significantly. In general, both kinds of hedgerows showed more significant effect on the total nutrients of soil than on available nutrients and V. zizanioides affected the redistribution of soil particles and nutrients more significantly than L. leucocephala did.In2011, the effect of hedgerows on soil nutrient became more complex. On the whole, soil organic matter, total nitrogen and phosphorus, available nitrogen and phosphorus, total potassium reduced. But available potassium increased greatly. Hedgerows’effect on soil available nutrient was greater than on the total nutrient. In the slope distribution, only total phosphorus’s distribution was similar as that in2010. Overall, soil nutrient contents and their slope distribution in2011were abnormal and long-term monitor is needed to explore the effect of hedgerows on soil nutrient characteristics. (3) In sloping cropland of10°,15°, V. zizanioides and L. leucocephala both improved the content of bacteria, fungi, actinomyces, microbiological carbon, microbiological nitrogen, and organic matters in soil. The content of bacteria, microbiological carbon and microbiological nitrogen increased more than50%. In general, V. zizanioides improved soil microbiological indicators mentioned above more significantly than L. leucocephala did. Especially the content of microbiological nitrogen was30%higher in V. zizanioides plots than that in L. leucocephala plots at the same declivity. The slope declivity also affected the soil microbiological indicators mentioned above significantly. These indicators increased as the slope got more moderate. These soil microbiological indicators showed a similar distribution tendency as soil nutrient in2010:increasing progressively from the top to the bottom of slopes and enriching before hedgerow lines. The content of microbes demonstrated remarkable positive relation with soil organic matters, total nitrogen and available potassium.(4) Under the same site, M. sativa, V. zizanioides, C. dactylon and P. notatum planted for1year had a significant effect on bulk density, porosity of surface soil layer (0-30cm), but they had almost no influence on soil particle composition and showed small influence on nutrients content of surface soil layer (0-20cm). Compared with control plots, soil bulk density decreased and total porosity increased in the4herbal plots. So soil aeration and water permeability had improved in herbal plots, especially in M. sativa Plot. In herbal plots, surface soil bulk density showed the opposite order of RLD (root length density) as0-10cm layer<10-20cm layer<20-30cm layer, but total porosity had the same order of RLD, shown as0-10cm layer>10-20cm layer>20-30cm layer. According to Spearman correlation analysis, roots of L≤0.5mm and L>5mm showed significant contributions to physical indicators mentioned above. Total nitrogen and available nitrogen showed significant correlations with RLD of all diameters, but other nutrients almost showed no correlation with RLD.(5) Under the same site, after1year’s plant, the root systems of M. sativa, V. zizanioides, C. dactylon and P. notatum could significantly strengthen anti-shear ability of root-soil complex in the surface soil layer (0-10,10-20,20-30cm), respectively. Among them the effect of V. zizanioides was the most significant:at100-400kPa vertical load, the anti-shear strength was improved117%-195%as compared with control, and the increase percentages of M. sativa, P. notatum and C. dactylon were18%-125%,18%-186%,21%-68%, respectively. In the plots of control, V. zizanioides, C. dactylon and P. notatum, anti-shear strength parameters (internal friction angle ψ, cohesionc) showed as:the soil layer got deeper, the value got smaller. The tendency in M. sativa plot was on the opposite. The root system showed stronger effect on the anti-shear ability of complex than on soil water content. The anti-shear ability of root was the key factor that improved root-soil complex anti-shear ability. The root of P. notatum had stronger anti-shear strength than that of M. sativa of the same diameter. Various diameters of RLD showed strong correlation with RDW (dry wet of root). But RLD showed no relation with φ andc, so as RDWto φ andc.(6) Under the same site,1year after the plant, root systems of M. sativa, V. zizanioides, C. dactylon and P. notatum significantly strengthened the anti-scourability of root-soil complex from the surface soil layer (0-30cm), respectively. At different time points of scour experiment, the sand contents of runoff in the herb plots were lower than that of control plot, and showed the order as V. zizanioides<P. notatum<C. dactylon<M. sativa. The changing process of M. sativa and V. zizanioides fit a quadratic function very well, while that of the CK and P. notatum area fit a power function very well. The4kinds of herbs all improved the anti-scouribility of soil, especially V. zizanioides and P. notatum. The soil anti-scouribility ANS and flashing time t fit a quadratic polynomial very well with a correlation coefficient. Among them, V. zizanioides and P. notatum fit the best. The root system of the4kinds of herbs all demonstrated very good effect on improving the anti-scuoribility of surface purple soil and V. zizanioides had the best effect with a277.53%increasing percentage, followed by P. notatum. According to Spearman correlation analysis, RLD had a significant positive correlation with ANS, and root surface area had an extremely significant positive correlation with ANS.In gentle slope of purple soil region, L. leucocephala and V. zizanioides hedgerows had beneficial effects on surface soil physical and chemical properties, microbial properties, and mechanical properties. As an ecological management measure to sloping cropland, hedgerow could be widely used. In the initial stage, V. zizanioides had a significant effect on soil properties mentioned above, which might contribute to the blocking effect of its thick stem and leaves and the decomposition of fallen leaves, the network holding and reinforcement effect of developed root system. The ability of M. sativa for soil fixation and water conservation was weaker than V. zizanioides. But its root is deep and it also had good effects on the physical and chemical properties of surface soil. M. sativa is a good forage grass, and it is suitable for pastoral areas in slope land management used as hedgerow plant. The root of P. notatum is small and only distributed in shallow soil layer. But its ability in soil conservation and anti-scour was second only to V. zizanioides. Therefore, P. notatum is suitable for soil conserve in shallow landslide area. The effects of hedgerow on the properties of purple soil in sloping cropland are a complicated process. So a long-term and all-round monitor and multidisciplinary, thorough research are necessary.