Characteristies Of Pore Size Distribution And Change In Upland Red Soil Under Long-term Manure Application

Soil porosity is the interstices within soil, and together with the soil solid phase composed of three dimensional structure of the soil. Soil pores directly affect the permeability and water conservation in soil and the stretch of crop root to soil space, and indirectly affect soil fertility and crop yield. Soil pores play an important role in agricultural production. Many of previous researches showned that improves the total porosity in soil under long-term application of manure fertilizers. However, the research result of long term fertilization experiment in Qiyang, Hunan province found that total porosity of red soil with manure application decreased. This abnormal phenomenon is firstly found in upland soils of China. The reason why soil total porosity decreased with the increase of organic matter content needs further research and discussion. This paper collected soil samples included the initial soil (O) in1990s and the soils from22years of long-term continuously non-fertilizer (CK), chemical fertilizer(NPK), NPK plus manure (NPKM), manure alone (M) under long-term fertilization experiment on red soil in Qiyang, Hunan province, and analysis change of pore size distribution, composition of aggregates and cements of tested soil, and discusses the mechanism of change process about distribution of composition of soil aggregates and soil cements in soil pore. The main results are as follows:(1) The different pore size distributed much uniform in the initial soil, however, after22years long term different fertilization, the number of>100μm soil pores increased, and the number of<100μm soil pores decreased, especially the decrease of application of manure (NPKM, M) on<100μm pores was higher than non-application of manure (CK, NPK). Pore structure parameters shows that the soil pore in red soil trend to be large pore under long term fertilization farming, especially the application of manure (NPKM, M) increased the width and specific pore volume of the soil pores.(2) The volume percentage of different size soil aggregates was measured by the laser particle-size, the results showed that the volume percentages of red soil aggregates distribution volume were:(2～53μm)>(<2μm)>(53～250μm)>(250～2000μm), the main part of soil aggregates was the2～53μm aggregates, its content reach to59.56～66.93%. Long term application of manure (NPKM, M) were significantly reduced the content of<2μm aggregates and increased the content of53～250μm and250～2000μm aggregates.(3) Long term application of manure (NPKM, M) increased the quantity of soil TOM and LOM, however, single application of chemical fertilizer or no fertilizer decreased the number of LOM. NPKM and M increased the percentage of LOM33～167the total labile organic matter, and reduced the percentage of LOM167～333the total labile organic matter, CK and NPK processing is just the opposite.(4) The content of different forms of iron and aluminum oxide in red soil after long-term different fertilization showed an upward trend. The degree of activation and complexing of iron and aluminum oxide were significantly increased in the fertilization treatments, and essentially keep unchanged in no-fertilizer (CK) treatments.(5) By using the method of changes the content of soil aggregates with removal and non removal soil organic matter, the result showed that organic cementing material of red soil mainly cemented<6μm micro-aggregates to form6～160μm aggregates, and the increment of6～160μm aggregates is proportional to the content of soil organic matter, in addition, the6～160μm aggregates will effectively blocked the10～100μm pore leads to the decrease of total porosity of soil; soil inorganic cementing material (iron and aluminum oxide) mechanism on formation of soil aggregates is more complicated.(6) Direct casue leading to reduced soil porosity under long term application of manure was the reduce of<0.01μm and10～100μm pores and the increase of specific pore volume in soil, but the>100μm pores was not directly cause. Path analysis showed that the distribution and variation of soil pore are mainly controlled by the<2μm,2μm～53,53～250μm aggregates, and250～2000μm aggregates have no direct effect; Soil cementation material is mainly Feo, MLOM and LOM33-167percentage of the total labile organic matter, Fep/Fed, LOM33percentage of the total labile organic matter and TOM have a direct effect on composition of soil aggregates. Soil total porosity reduction process under long term application of manure has two main ways:(1) increase of MLOM→decrease of<2μm aggregates→decrease of<0.01μm pores→decrease of soil total porosity;(2) increase of TOM→increase of53～250μm aggregates→increase of specific pore volume of the soil pores→decrease of soil total porosity.