Objectives The soil of dryland areas of the Loess Plateau is often barren, the heat and precipitation resources can not be fully utilized during summer fallow period. Therefore, a field experiment was conducted to investigate effects of green manure crop varieties and planting patterns on their biomass, nutrient contents, soil nutrient contents, soil enzyme activities, soil microbial biomass carbon (SMBC) and dissolve organic carbon (DOC) to determine the best green manure crop variety and planting pattern, which can be regarded as the theoretic base and technical support for the development of sustainable agriculture in the Loess Plateau.
Methods An experiment with randomized complete block design was conducted. The bare fallow in summer was used as control, two factors were green manure varieties (Mung bean, Huai bean, Hairy vetch and Rape)and planting patterns (after wheat harvest and intercropped with wheat), counting to 9 treatments. The green manure was harvested in full blooming stage and the yield of fresh grass was weighted, the N, P and K contents were analyzed. The root samples were collected inside volume 20 cm × 20 cm × 20 cm at the bottom of 50 plants in each plot and weighted, soil samples (0-20 cm deep) were collected 20 days after the green manure was returned into field, and the contents of nutrients, soil microbial biomass carbon (SMBC) and dissolve organic carbon (DOC), as well as the activities of main soil enzymes, were determined.
ResultsThe results indicated that the after wheat harvest had greater biomass than intercropping, the Huai bean and rape treatments had greater biomass than other green manures in the after wheat harvest, and the biomass of rape treatments was significantly increased in contrast with other green manures in the intercropped with wheat. The changes of green manure nutrient contents were similar to those biomass in different planting patterns. Huai bean provided the highest amounts of nitrogen and phosphorous, while rape offered the highest amounts of phosphorous and potassium through incorporation in the after wheat harvest, and the amounts of nitrogen, phosphorous and potassium through incorporation in the intercropped with wheat were rape > hairy vetch > Mung bean > Huai bean. The rape treatment significantly increased soil organic matter (SOM), available phosphorous and available potassium compared with the fallow. After 2 years of the after wheat harvest, the Huai bean treatment enhanced soil organic matter(SOM), total nitrogen, mineral nitrogen, available phosphorous and available potassium contents in the depth of 0-20 cm by 12.4%, 22.2%, 95.9%, 28.6% and 11.2%, respectively, compared with the fallow. Incorporation of green manure during summer fallow period increased soil microbial biomass carbon (SMBC) content and dissolve organic carbon (DOC) content compared with the fallow, and the rape treatments significantly increased soil microbial biomass carbon (SMBC) and dissolve organic carbon (DOC) in the intercropped with wheat. There were no significant differences of soil microbial biomass carbon (SMBC) between the green manure treatments in the after wheat harvest, however, the contents were greater than those of the fallow (P < 0.05). Green manure in the after wheat harvest enhanced soil enzyme activities compared with the fallow, and other intercropped with wheat green manures increased soil enzyme activities except the Mung bean treatment which decreased the enzyme activities of urease and catalase. The significant correlations between soil properties and green manure biomass indicated that the soil fertility was determined by the amount of green manure biomass to soil.
Conclusions The Huai bean and rape treatments could significantly increase soil nutrient contents and soil enzyme activities and they were better than Mung bean and hairy vetch. As for the planting patterns, the after wheat harvest was better than the intercropped with wheat. Huai bean and rape as green manure applied after wheat harvest can effectively improve soil quality in dryland of the Loess Plateau.
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