Objectives We studied the soil enzyme activities, nutrient contents and crop yield after 18 years consecutive different fertilizations, so as to provide a basis for rational fertilization and farmland cultivation in the farming-pastoral ecotone.
Methods A long-term positioning experiment was carried out in Dryland Experimental Station, Academy of Agricultural and Forest Sciences, Inner Mongolia. The experiment started since 2004, the cropping system was potato-rapeseed-naked oats rotation, and one crop per year. The experiment was composed of eight treatments, including: CK (without fertilization), N (nitrogen fertilizer), NP (only N and P fertilizer), NK (only N and K fertilizer), PK (only P and K fertilizer), NPK (N, P and K fertilizer), M (sheep manure), and NPKM (NPK combined with sheep manure), all the three crops were under the same fertilizer input scheme, and the crop was naked oats in 2021. After harvest, naked oat yield was investigated. Soil sample was taken from the topsoil (0−20 cm) of each plot for the determination of phosphatase (PA), sulfatase (SU), β-glucosidase (BG), β-xylosidase (BX), α-glucosidase (AG), β-cellobiosidase (BCBH), N-acetyl-glucosaminidase (NAG), L-leucine aminopeptidase (LAP), and soil organic matter (SOM), alkali-hydrolyzed nitrogen (AN), available phosphorus (AP), readily available potassium (AK), and pH. Principal component analysis and redundancy analysis were used to explore the distribution characteristics of soil enzyme activities and their relationship with soil nutrients and yield.
Results All the fertilizer treatments significantly increased the yield and the contribution rate of fertilizers to yield. The yield increment and fertilizer contribution rate were in the descent order of NPKM>NPK>NP≈M>NK>N>PK>CK, with the yield increment relative to CK of 140.5%, 108.7%, 76.3%, 71.1%, 57.5%, 48.2% and 20.8%, respectively, and the fertilizer contribution rate of 58.4%, 52.1%, 43.3%, 41.5%, 36.4%, 32.5% and 17.0%, respectively. Long term fertilization effectively increased soil organic matter and available N, P and K content in different degrees, decreased pH. NPKM treatment was recorded the highest increment in OM and AN, AP, and AK, which were 5.35, 2.29, 10.07 and 4.89 times of CK. NPK and NPKM treatment were recorded the lowest pH. Long-term fertilization significantly increased soil enzyme activities. Both NPK and M treatments significantly increased activities of AG, BG, BX and BCBH (involved in carbon cycling), LAP and NAG (involved in nitrogen cycling), and PA and SU. In NPKM treatment, all the tested soil enzyme activities were significantly improved than those in NPK and M treatments, and the activities of AG, BG, BX, BCBH, LAP, NAG, PA and SU were 1.71, 1.87, 2.05, 2.11, 1.97, 2.24, 2.24, and 3.11 times of those in CK. The soil AN, SOM and AP were the key factors causing the variation of soil enzyme activity, explaining 75.1%, 5.7% and 4.6% of the variation of enzyme activity, respectively.
Conclusions In the farming-pastoral ecotone under rotation of potato-rapeseed-naked oat, balanced NPK fertilization can increase crop yield and the contribution rate of fertilizer to yield, and enhance soil nutrient content and enzyme activity, reduce soil pH in the long run. Merely applying sheep manure can improve soil fertility as well, but cannot increase crop yield as effective as the NPK fertilization. The combination of NPK fertilizer and sheep manure exhibits promotion effect that is significantly better than the separate application of them.
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