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A Preliminary Study on the Determination of the Fertilization Tolerance of an Entisol in the Yuanmou Dry-Hot River Valley Based on Soil Qualities in Plot Scale

Using field slope farmland plots, this study planted the typical crop of maize (Zea mays L.) and investigated the effects of varied chemical fertilizer (organic compound fertilizer of potassium nitrate, containing 17% each of nitrogen, phosphorus, and potassium) application levels (0.5 times the common fertilizer amount (CK, 0.75 t·hm−2 to 2.5 CK) on the soil fertility in the Yuanmou dry-hot River Valley. The results showed that the soil chemical properties, microbial properties, and enzyme activities increased with the increase of fertilizer application levels from CK to 2.0 CK. However, a declining trend showed both under 0.5 CK level and the 2.5 CK level, and higher in fertilizer application level 3 (1.5 CK) and level 4 (2.0 CK) compared to level 1 (1 CK). Soil chemical properties, microbial properties, and enzyme activities in five-degree slope cropland topsoil were higher than these in 10-degree slope cropland topsoil. Five parameters (available N, nitrifying bacteria, inorganic phosphorus bacteria, organic matter, and invertase) in five-degree slope cropland and three parameters (organic matter, ammonifying bacteria, and total P) in 10-degree slope cropland, which had the greatest weight in the principal components analysis, were selected to calculate the soil quality index (SQI). The SQI calculated by integrating all critical parameters indicated that the highest SQI values were found in fertilizer levels 1.5 CK (0.71) and 2.0 CK (0.69), followed by CK (0.64), and the lowest were found in 0.5 CK (0.62) and 2.5 CK (0.61) in five-degree slope cropland soil. The highest SQI values were found in fertilizer levels 1.5 CK (0.26) and 2.0 CK (0.29), followed by CK (0.23), and the lowest were found in 0.5 CK (0.14) and 2.5 CK (0.20) in 10-degree slope cropland soil. The final SQI values implied that the fertilization treatment 2 (CK), fertilization treatment 3 (1.5 CK) and fertilization treatment 4 (2.0 CK) could improve the soil fertility, whereas the fertilization treatment 1 (0.5 CK) and fertilization treatment 5 (2.5 CK) could decrease the soil fertility. In view of the impact of slope, the soil qualities of five-degree slope cropland of five fertilization treatments were higher than in 10-degree slope cropland. The SQI values in five-degree slope cropland soil were found higher than the SQI values in 10-degree slope cropland soil by 68.65%, 64.20%, 62.22%, 57.46%, and 67.01%, respectively. For this study, the range of fertilization tolerance was 0.75–1.50 t·hm−2 (organic compound fertilizer of potassium nitrate) in 10-degree slope plot scale and 0.75–1.13 t∙hm−2 in five-degree slope cropland soil.