| Cover crop is a kind of forage grass or other plants that are artificially planted around the target crop,which is also called green manure plant in China.Planting cover crops in orchards is of great significance to reduce soil erosion,improve soil ecological environment and promote the quality and sustainable development of fruit industry.At present,the research of orchard cover crops mainly focuses on the influence of single cover crop planting on the physical and chemical properties of orchard soil,there is a lack of systematic research on the influence of cover crop diversity,such as,the influence of mixed planting patterns of different cover crops on aboveground productivity and orchard soil characteristics.Therefore,in this paper,two grasses（Bentgrass and Tall fescue）,two legumes（White clover and Red clover）and two herbs（Chicory and Ribwort）were selected,which grow well in southwest China.A total of 3functional groups and 6 cover crop species were studied,establish cover crop communities with different levels of diversity in citrus orchard.The aboveground biomass and nutrient contents of different cover crops,soil moisture,temperature,total nitrogen,alkali-hydrolyzed nitrogen,organic matter,urease,alkaline phosphatase and other soil physicochemical indexes and soil enzyme activity indexes of 0-20cm soil layer were measured continuously,so as to explore the relationship between aboveground productivity and soil characteristics.Combined with principal component analysis and grey correlation analysis,the comprehensive evaluation was carried out to select the suitable mixed planting types of cover crops in citrus orchard,in order to provide theoretical basis and technical model for reasonable grass planting system and sustainable orchard production.The main findings are as follows:（1）There were significant differences in aboveground biomass and nutrient accumulation among different cover crops.Compared with monoculture treatment,the aboveground biomass of cover crops increased by 17.16%on average,and the aboveground biomass of 2 legumes and 2 herbs mixculture treatment（L1+L2+H1+H2）was the highest,which was 11477.43kg·hm-2,increased by 37.49%compared with monoculture treatment.In the mixculture treatments of different functional species,the herb cover crops were in a dominant position,accounting for 20.27-87.12%of biomass,followed by legume cover crops,accounting for 5.02-80.35%,and grass cover crops accounted for the least.Compared with monoculture,cover crop mixculture could reduce weed yield significantly.The accumulation of carbon,nitrogen,phosphorus and potassium in L1+L2+H1+H2 treatment was also higher,which were 4987.96kg·hm-2,435.85 kg·hm-2,65.43 kg·hm-2 and 413.57 kg·hm-2,respectively.The fit degree between aboveground biomass and nutrient content of different cover crops was better,which had a positive linear correlation with plant phosphorus content,but had no significant linear correlation with plant nitrogen content and potassium content.（2）Different cover crop treatments could improve soil physical and chemical properties and enhance soil enzyme activities.Compared with CK treatment,different cover crop treatments could reduce soil temperature in spring,summer and autumn,and increase soil temperature in winter.It can increase soil moisture by 2.29-6.15%and reduce soil water content by 16.20-47.27%.Compared with CK treatment,soil total nitrogen,alkali-hydrolyzed nitrogen,available phosphorus,available potassium and organic matter contents increased by 5.29%,6.55%,6.35%,10.44%and 5.37%,respectively,and soil p H decreased by 2.88%.The activities of soil urease,invertase,catalase,alkaline phosphatase and dehydrogenase were increased by28.60%,21.83%,34.47%,29.26%and 19.21%,respectively.There was a linear correlation between soil enzyme activities and soil fertility factors under different cover crop treatments.Soil sucrase and alkaline phosphatase were positively correlated with soil total nitrogen,alkali-hydrolyzed nitrogen,available phosphorus and organic matter,and negatively correlated with soil p H.In general,cover crops with 2-4 species richness and 2-3 functional groups richness had significant effects on soil physicochemical properties.（3）The relationship between aboveground biomass of cover crops and species richness and functional group richness was characterized by a one-peak curve.When the number of species was 4 and the number of functional groups was 3,the biomass of cover crops reached the peak.The aboveground weed biomass decreased with the increase of cover crop richness and functional groups richness.The contents of carbon,nitrogen,phosphorus and potassium in the aboveground of cover crops increased first and then decreased with the increase of the number of cover crop richness and functional groups richness.Compared with the single functional group,the mixed multifunctional group could increase the aboveground nitrogen content and phosphorus content by 2.13-5.53%and 7.49-13.70%on average,but there were no significant differences in aboveground potassium content among different species richness and functional group richness.Aboveground nutrient accumulation followed 4 species>3species>6 species>2 species>1 species.Compared with the single functional group,the aboveground accumulation content of carbon,nitrogen,phosphorus and potassium increased by 15.02-20.66%,20.64-22.87%,22.05-33.90%and 12.70-24.26%,respectively.（4）With the increase of species richness,soil total nitrogen,alkali-hydrolyzed nitrogen,available phosphorus and organic matter content increased first and then decreased,but there was no significant difference in soil p H,soil available potassium and soil moisture contents between monocultural and mixed sowing.Soil nutrient content varied with functional group richness followed 2 species>3 species>1 species.Compared with single functional group,soil total nitrogen,alkali-hydrolyzed nitrogen,available phosphorus and organic matter content increased by 3.95-4.29%,7.04-8.68%,1.46-6.73%and 2.34-5.58%,respectively,by the treatments of mixed multiple functional groups.There were no significant differences in the activities of sucrase,alkaline phosphatase,catalase and dehydrogenase among different species richness（P<0.05）.Soil urease,invertase and alkaline phosphatase activities of single functional group richness were significantly lower than those of other functional groups.Compared with single functional group richness,soil urease,invertase and alkaline phosphatase activities of multiple functional groups were increased by 1.61-2.51%,6.53-7.31%and 2.48-2.86%,respectively.The correlation between soil characteristics and functional group richness was higher than that between species richness.（5）The comprehensive evaluation results based on principal component analysis and grey relational degree analysis are basically consistent.The comprehensive evaluation results of principal component analysis and grey correlation analysis showed that the comprehensive scores of tall fescue+ribwort（G2+H2）,bentgrass+chicory（G1+H1）and bentgrass+tall fescue（G1+G2）treatments were lower than those of monoculture treatment.The herb cover crops was beneficial to increase the benefit of mixculture.When species richness was 3-4 and functional group richness was 2-3,the mixculture effect of cover crops was optimal.Combined with the comprehensive evaluation ranking results of principal component analysis and grey correlation analysis,the optimal treatment of mixculture effect was tall fescue+white clover+chicory（G2+L1+H1）and bentgrass+tall fescue+chicory+ribwort（G1+G2+H1+H2）.In conclusion,different cover crop mixculture patterns had different effects on aboveground biomass,soil physical and chemical properties,and soil enzyme activities.There was a certain relationship between cover crop diversity and aboveground productivity,soil characteristics.The comprehensive evaluation showed that when the species richness of cover crops was 3-4 and the functional group richness was 2-3,the correlation degree was the maximum,which was the ideal model for the cover crop mixculture in citrus orchard.In this study,the optimal mixculture treatments were tall fescue+white clover+chicory and bentgrass+tall fescue+chicory+ribwort. |
