The Carbon Balance In Maize-soybean Relay Strip Intercropping System

With the global climate changes scenarios, the intensive agricultural production and food security are facing enormous challenges. Therefore, using agronomical cultivation techniques to produce more food as well as reduce atmospheric carbon content is vital for agriculture sustainable development, which has become one hot field in crop cultivation science and ecology research. To study the carbon balance of maize-soybean relay strip intercropping (RI), we carried out a series of field experiments in three counties (Renshou, Lezhi and Ya’an) of Sichun province from 2012 to 2013. We explored the carbon sequestration of crop, soil carbon emissions and soil organic carbon changes in maize-soybean relay strip intercropping (RI1) compared with those in maize-soybean relay intercropping (RI2), maize monoculture (MM), soybean monoculture (SM) and fallow land (FL). In this study, we also studied the ecosystem goods production and soil fertility maintenance of maize-soybean relay strip intercropping. The results are following.1. The net primary productivity of crops was significantly higher in RI1 than that in MM, SM or RI2 (P<0.05). The net photosynthetic rate and net primary productivity of maize in RI1 and RI2 were not significantly changed compared to those in MM, but those of the relay-intercropped soybean plants were significantly reduced compared with SM. The total net primary productivity in RI1 was approximate 0.94 kg C m-2 in the growing season, which was 5.96% higher than that by RI2,23.9% higher than that by MM and 217.6% higher than that by SM. The amount of carbon transported to the soil via crops was also significantly higher in RI1 than that in monoculture and RI2 (P<0.05). If aboveground straw taken away from the field, the value was about 118.5g m-2, which was significantly higher than RI2 (13.6%) and MM (59.1%); and if not, the value reached at 594.4 g C m-2, which was 7.4% higher than RI2 and 26.9% higher than MM, respectively. Regression analysis showed that the higher net primary productivity of crops in relay intercropping may be attributable to the higher leaf area index of crops at the co-growth stage and the longer growth duration of crops. However, stepwise regression analysis showed that only the leaf area index of crops at the co-growth stage was significantly positively correlated with the net primary productivity (y= 173.83+188.4x, R2=0.941, P<0.01).2. The mean soil respiration rate in RI1 was 268.4 mg m-2 h-1 during the growing season, and was significantly lower than that in MM (-11.8%) in co-growth stage (P<0.05), however, it was not significantly different with RI2. Compared with MM, RIl possessed significantly lower soil temperature (T) and photosynthetically active radiation (PAR) below the crop canopies, but higher soil moisture content (SMC). We found that the relative loss of soil respiration rate in RI1 was significantly positively correlated with the relative loss in the T and the PAR, but was significantly negatively correlated with the relative increase in SMC, which indicated that the reduction in soil respiration rate in RI1 may be attributable to the reduction in the T and PAR, and the increase in the SMC caused by the spatial layout of crops compared with monoculture. In addition, there existed spatial heterogeneity of soil respiration rate in RI1, showing higher in maize strips than in soybean strips or the inter-strips. Such spatial heterogeneity was significantly positively correlated with the spatial heterogeneity of root biomass in RIl (y= 327.51+1.531x, R2=0.827, P<0.001), which indicated that spatial heterogeneity of soil respiration rate in RI1 could be attributed to the heterogeneous spatial distribution of crop roots.3. According to the results from the three experiment sites in the second year, the soil organic carbon content (SOC) in RI1 was higher than that in monoculture, with 9.14% higher than MM and 5.76% higher than SM, respectively. However, the SOC and soil physicochemical property in RI1 were not significantly different with RI2. Soil fertility and physical properties in RI1 were notably or slightly higher than monoculture, with soil organic matter (SOM) by 7.47%, soil alkali-hydrolyzable nitrogen (AN), available phosphorus (AP), available potassium (AK), and soil moisture content (SMC) by 14.79%, 22.54%,12.47% and 10.26% respectively, but soil bulk density (SBD) lower by 6.58%. These results indicated that soybean intercropped with maize had more advantages on soil fertility maintenance than monoculture did. The SOC was significantly positively correlated with the AN, AP and SMC, but was significantly negatively correlated with the SBD. However, stepwise regression analysis showed that AP was introduced firstly to the model, and AN was followed (SOC=0.610+0.325AP+0.199AN, R2=0.857, P<0.001), which indicated that changes in soil AP and AN content may be a major factor in the change of soil SOC in RI1. In addition, the SOC was also significantly positively correlated with the amount of carbon transported to the soil via crops, which indicated that the increase in the SOC may be attributed to the higher soil fertility maintenance ability and carbon input from crop residues in RI1 compared with monoculture.4. The carbon input for crops production was increased in RI1 and RI2 compared with monoculture. Such higher carbon input in RI1 and RI2 was attributed to the more input of fertilizers. However, the carbon sequestration of crops in RI1 was improved significantly, and the total net primary production (NEP) was improved by 41.6% and 10.7% in RI1 compared with that in MM and RI2, respectively. The NEP was analyzed to be significantly positively correlated with net primary productivity (NPP), which indicated that the increasing utilization of natural resources in intercropping systems could improve the carbon sink function of crops in farmland ecosystem.5. RI1 did not significant influence the yield and yield component of maize crop compared with MM, but significantly reduced soybean yield (-15.6%) and total biomass (-37.8%) compared with SM. As a whole, both the total yields and biomass output from RI1 were significantly higher than those from monoculture and RI2, which were 17.8% and 24.2% higher than MM,352.2% and 211.7% higher than SM, and 5% and 5.97% higher than RI2, respectively. Land equivalent ratio (LER) in RI1 was 1.83, which was significantly higher than RI2 (1.54). The total grain yield and biomass were significantly positively correlated with the leaf area index of crops at the co-growth stage and the growing time of crops, respectively, the bivariate regression equation for grain yield was Yield= 3026.186 LAI-5.04 TIME-2488.558, and for biomass was Biomass= 6010.756 LAI+1.836 TIME-3668.718.Based on the above findings, soybean relay intercropped with maize can improve net primary productivity of cropland, reduce soil respiration rate in co-growth period, as well as increase the content of soil organic carbon compared with monoculture. Meanwhile, this cropping pattern can also maintain soil fertility and improve crop yield.