| Other Abstract | Agricultural activity is the major source of greenhouse gases. Rational land use and effective agronomic measures are beneficial to increase agricultural soil fertility, store more organic carbon, and obtain high yield and reduce greenhouse gas emissions. Earlier studies have shown that cereal-cereal intercropping and cereal-legume intercropping have a significant impact on rainfed farmland productivity and carbon emissions, but there are few reports on how to respond under different nitrogen addition conditions, as well as the process and mechanism of species interactions. In particular, there is a lack of systematic exploration on the mechanism of competition theory and positive interaction theory in classical ecology.
Maize, wheat and soybean were used as experimental materials in this study, and the field experiment was carried out at the Agricultural Ecological Experimental Station on Yuzhong Campus of Lanzhou University from 2019 to 2020. There were two main treatment groups: 1) different intercropping treatments: maize-wheat intercropping (MW), maize-soybean intercropping (MS), wheat-soybean intercropping (WS), sole maize (SM), sole soybean (SS) and sole wheat (SW);2) different nitrogen levels: N1 (138 kg·ha-1), N2 (207 kg·ha-1) and N3 (276 kg·ha-1). The main results of the study are as follows:
1. Interspecific interaction conditions improved dry farmland productivity and water use efficiency. In the maize-wheat intercropping system and soybean-wheat intercropping system, wheat was the dominant species. Under intercropping conditions, wheat yield and water use efficiency (WUEY) increased, while maize and soybean yield and water use efficiency (WUEY) decreased. In the maize-soybean intercropping system, the yield and water use efficiency (WUEY) of both soybean and maize significantly increased, and the intercropping significantly improved the root morphological parameters such as dry matter accumulation, shoot and root biomass, leaf area ratio (LAI), root length density and other root morphological parameters of maize. The land equivalent ratios (LER) of maize-wheat intercropping, maize-soybean intercropping and wheat-soybean intercropping were in the range of 0.96~1.02, 1.02~1.13 and 0.99~1.13, respectively. These results indicated that cereal-legume intercropping was beneficial to improving water resource utilization of crops, and had higher yield advantages and water resource utilization efficiency than that of cereal-cereal intercropping.
2. Nitrogen addition significantly increased dry farmland productivity and water use under the background of interspecific interaction. The addition of nitrogen increased the yield of each crop, the dry matter accumulation, root length density and other morphological parameters of maize. Compared with N1, under N2 and N3 treatment the yield and WUEY of maize were increased by 15.28% and 16.78%, 17.97% and 14.97%, and soybean yield and WUEY were increased by 12.26% and 8.45%, 18.96% and 11.68% in MS, respectively;in MW, the yield and WUEY of maize were increased by 7.97% and 8.62%, 9.15% and 10.55%, and wheat yield and WUEY were increased by 8.88% and 13.86%, 14.87% and 18.36% respectively;in WS, wheat yield and WUEY were increased by4.88% and 11.01%, 14.64% and 16.06%, and soybean yield and WUEY were increased by 10.10% and 9.91%, 20.02% and 24.80%, respectively. And with the addition of nitrogen, the LER of each intercropping system also increased. The results showed that nitrogen addition was beneficial to improving crop productivity and water use efficiency under intercropping conditions, that is, to improving the interspecific intercropping advantages of crops.
3. The rhizosphere SOC of wheat increased significantly under cereal-cereal intercropping;meanwhile, the rhizosphere SOC of maize and wheat increased significantly under cereal-legume intercropping. Compared with monoculture, intercropping with soybean increased the soil organic carbon content in the rhizosphere of maize and wheat. The SOC of maize rhizosphere under MS was increased by 3.78%, and that of wheat rhizosphere under WS was increased by 9.54%. Intercropping decreased the SOC content in soybean rhizosphere soil. Maize-wheat intercropping significantly increased the soil organic carbon content of wheat rhizosphere by 2.81%, while decreased the soil organic carbon content of maize rhizosphere by 2.89%. This is because there were complementation, competition, and transfer of soil nutrients in the intercropping system, and different species had different ability to acquire resources. And under the same nitrogen gradient, intercropping with maize or soybean could increase its microbial biomass carbon content. This shows that intercropping with maize or soybean contributes to improving soil nutrients.
4. Nitrogen addition increased the SOC content in the rhizosphere of crops under the background of interspecific interaction. Compared with N1, under N2 and N3 treatment, the rhizosphere SOC of maize was increased by 3.40% and 14.16%, and that of soybean was increased by 4.19% and 11.33% in MS;in MW, maize and wheat rhizosphere SOC were increased by 6.78% and 17.81%, and 5.04% and 9.85% respectively;the rhizosphere SOC of wheat and soybean were increased by 4.18% and 7.04%, and 3.31% and 11.10% respectively in WS;for sole maize, rhizosphere SOC was increased by 5.31% and 14.15%, respectively;for sole soybean, rhizosphere SOC was increased by 2.74% and 11.52%, respectively;for sole wheat, rhizosphere SOC was increased by 5.40% and 10.33%, respectively. These results indicated that nitrogen addition was beneficial to the improvement of soil organic matter content, that is, under the condition of interspecific interaction, nitrogen addition was positively correlated with soil organic carbon.
5. Under the condition of cereal-legume intercropping, the content of N in the rhizosphere of legume decreased, while that of cereal increased;under cereal-cereal intercropping, the variation of N content in the rhizosphere of crops was different due to inter-specific competition. In terms of total nitrogen and inorganic nitrogen, intercropping with soybean significantly increased the total nitrogen and inorganic nitrogen contents of maize and wheat rhizosphere soil. The TN content of maize rhizosphere and wheat rhizosphere under MS were increased by 8.48% and 14.82%, respectively. However, the contents of total nitrogen, NO3- and NH4+ in the rhizosphere of soybean under intercropping conditions were significantly lower than those under sole soybean, and the TN content in the rhizosphere of soybean under MS and WS were decreased by 9.44% and 18.04% compared with soybean monocropping. Maize-wheat intercropping significantly increased the TN content of wheat rhizosphere soil by 8.71%, while decreased the TN content of maize rhizosphere by 7.75%. The results showed that intercropping was beneficial to increasing the contents of TN and inorganic nitrogen in the rhizosphere of wheat, but significantly decreased the content of N in the rhizosphere of soybean.
6. Nitrogen addition was positively correlated with the contents of TN and inorganic N in the rhizosphere under the background of interspecific interaction. Compared with N1, under N2 and N3 treatment, the rhizosphere TN of maize was increased by 8.55% and 9.98%, respectively, and that of soybean by 11.50% and 21.28%, respectively in MS;the rhizosphere TN of maize and wheat in MW were increased by 10.23% and 19.01%, and 15.07% and 23.52%, respectively;in WS, the rhizosphere TN of wheat and soybean were increased by 6.77% and 16.66%, and 8.21% and 19.06%, respectively;for sole maize, rhizosphere TN was increased by 8.04% and 16.75%, respectively;for sole soybean, rhizosphere TN was increased by 6.78% and 13.56%, respectively;for sole wheat, rhizosphere TN was increased by 6.69% and 23.29%, respectively. The results showed that the contents of total nitrogen and inorganic nitrogen in the rhizosphere were increased with the increase of nitrogen application rate. This indicates that too high level of nitrogen application is not conducive to the absorption and utilization of nitrogen fertilizer by plants, and it is easy to cause the loss of nitrogen fertilizer due to the conversion of more inorganic nitrogen in soil.
7. Under the condition of interspecific interaction, the total soil carbon emission of the whole system decreased significantly;interspecific interaction also significantly improved the carbon emission efficiency of crops. The soil respiration rate showed a seasonal change during the growth period from 2019 to 2020, with an unimodal trend of increasing first and then decreasing. The soil respiration rate reached the peak value in summer, and there were significant differences among different nitrogen levels and intercropping patterns;in winter, the soil respiration rate was the lowest, and there was no significant difference among different treatments. This is because soil respiration rate is affected by a variety of ecological factors, among which soil respiration rate and soil temperature are highly correlated. Affected by the size of biomass, the order of soil respiration rate values under monocropping system was as follows: SM >SW >SS. For maize, intercropping with soybean increased its soil respiration rate, which was contrary to intercropping with wheat. For soybean, the soil respiration rate under wheat-soybean intercropping system was lower than that of soybean monoculture. And for wheat, the soil respiration rate under intercropping system was higher than that of sole wheat. Compared to maize monoculture, the decreases of total carbon emissions of maize-wheat intercropping and maize-soybean intercropping were 17.50% and 18.33%, respectively. Compared to wheat monoculture, the decrease of total carbon emission of wheat-soybean intercropping was 0.98%. And intercropping significantly improved the carbon emission efficiency of crops compared with monoculture. The carbon emission equivalent ratio of maize-wheat intercropping ranged from 0.96~1.02, maize-soybean intercropping ranged from 1.01~1.08, and wheat-soybean intercropping ranged from 1.04~1.09. These results indicated that maize-wheat intercropping and maize-soybean intercropping system can effectively reduce the carbon emission during crop growth and development, and had the potential to produce more grain yield. Under the same nitrogen gradient, intercropping significantly reduced the total carbon emission of the whole system.
8. Nitrogen addition increased the soil carbon emission rate and total soil carbon emission under the background of interspecific interaction. The results of this experiment showed that with the increase of nitrogen application, the soil respiration rate showed an increasing trend, and the total carbon emission also increased. On the whole, compared with N1, under N2 and N3 treatments, the total carbon emissions in maize area were increased by 18.68% and 23.72%;the total carbon emissions in soybean region were increased by 10.42% and 22.56%;and the total carbon emissions in wheat area were increased by 9.41% and 22.09%. With the addition of nitrogen, the carbon emission equivalent ratio of soybean-wheat intercropping system was decreased;compared with N1, the carbon emissions equivalent ratio of maize-wheat intercropping under N2 and N3 treatments were increased by 7.29% and 3.20%, respectively;the carbon emission equivalent ratio of maize-soybean intercropping system was not significantly different under different nitrogen gradients. In conclusion, for the maize-wheat intercropping system, the carbon emission equivalent ratio was the highest under N2 gradient, and the effect of increasing production and reducing emissions was the best. With the addition of nitrogen, soil carbon emission rate and total carbon emission increased gradually, and interspecific interaction had the best effect on yield increase and emission reduction under the condition of moderate nitrogen addition.
These results suggest that maize-soybean intercropping system and wheat-soybean intercropping system combined with reduction of nitrogen fertilization is an effective mode of reducing carbon emission and increasing carbon emission efficiency. This shows that the combination of intercropping mode and rational nitrogen application can not only ensure crop yield, but also reduce soil carbon emission, which is conducive to sustainable agricultural development. Among them, under the N2 gradient, the maize-soybean intercropping and wheat-soybean intercropping modes had the best effect, that is, under the normal nitrogen application cndition, introducing the legume intercropping mode into the field planting system is a feasible way to stabilize crop yield and reduce emission. |
