Effects of 25-yr located different fertilization measures on soil hydraulic properties of lou soil in Guanzhong area

Soil moisture is a main limiting factor for crop growth and vegetation reconstruction in the arid and semi-arid areas of the world and it is a key factor for agricultural sustainability and soil productivity. Changes in soil hydraulics and function are major aspects for assessing whether long-term fertilization is beneficial for soil sustainable development or not. The objectives of this study were to determine the soil water retention characteristics of different fertilization treatments based on the long-term located experiment of soil fertility at "the Chinese National Soil Fertility and Fertilizer Efficiency Monitoring Base of Loessial Soil" established in the autumn of 1990 in Yangling, Shaanxi, China, and to clarify the differences of soil physical properties affected by the fertilization. The experiment was consisted of six treatments: abandonment (LH), fallow (tillage， without planting, XX), no fertilization (CK), single application of nitrogen (N), nitrogen, phosphorus and potassium (NPK), organic manure + NPK (MNPK), and LH and XX are not fertilized, and weeds were regularly removed at XX every year. The soil water retention curve, saturated hydraulic conductivity and soil strength were analyzed and determined by adopting the surface undisturbed soil. The results showed that: 1) Long-term fertilization treatment had significant effects on soil organic carbon, saturated hydraulic conductivity and soil strength (P<0.05). Compared with CK, N and NPK, MNPK significantly (P<0.05) increased soil organic carbon, saturated hydraulic conductivity and porosity and reduced soil bulk density and soil strength. 2) Soil water retention curve showed a significant difference between treatments, and soil water holding capacity was: XX < N ≈ NPK < CK < MNPK < LH. Soil water holding capacity of MNPK was increased by 2.57%, 3.33% and 7.34% over that of the CK, N and NPK, respectively. The parameters of V-G model between different treatments showed that there were slightly significant differences in residual water content (θr), saturated water content (θs) and reciprocal of inlet air (a). θr is the largest in MNPK, and the smallest in XX; θs was the largest in N, followed by MNPK, and the smallest at CK. Inlet air (1/a) was the largest in XX and the smallest in LH. 3) The pores of different treatments were mainly composed of an equivalent pore diameter > 9 μm, and the ratio of macropores in different treatments was higher than that of medium pores and small pores, ranging from 22.3% to 30.2%. Compared with medium pores, the ratio of small pores was more. There was the largest ratio for the small pores at the LH, followed by the MNPK, and the XX. In summary, MNPK can improve soil structure and soil water holding capacity, reduce soil bulk density and soil strength and enhance soil stability, which is helpful for crop growth and yield, and is a suitable farming measurement in semi-arid Guanzhong area.