| Paddy-upland rotation system has been adopted in Yangtze River Basin for more than 1000 years, which is one of the most important cropping systems to increase grain production and ensure food security. The paddy-upland rotation system is an ideal model for investigating the interaction between soil processes and rhizosphere processes, because of the seasonal soil drying-wetting transitions and different upland crops with various rhizosphere processes involved in the cropping system. However, balance and movement of phosphorus (P) in this system are not clear. To understand the mechanism of P balance and availability is important for optimizing this cropping system and increasing soil P acquisition efficiency. Here, my work focused on P issues in the paddy-upland rotation system, and the related studies were conducted with two different parts:data mining and case study. Firstly, by systematic collection and comprehensive analysis of large amounts of data, we estimated the evolution of P balance in Yangtze River Basin, and quantified the historical contribution of paddy-upland rotation system to the food production and fertilizer inputs in Yangtze River Basin. We also discussed the problems in P balance and proposed the possible solutions in this part. The problems in P balance in such cropping system were also discussed and the possible pathways for overcoming the problems were proposed in this study. Secondly, with field and pot experiments, we further investigated the crop productivity and P balance in three different cropping systems:paddy-upland rotation system, continuous rice system and dryland rotation system. In these experiments, the P movements along soil profiles were monitored. The P use efficiencies in main crops with various rhizophere processes were compared. The results revealed the effects of drying-wetting cropping and plant rhizosphere processes on soil P balance, mobilization and interaction mechanisms between them, which are critical for optimizing P use efficiency in the rhizosphere process-based crop management under a specific cropping system. The main results and conclusions drawn from this study were listed as follows:(1) The balance and loss of P in Yangtze River Basin, which are the main research contents for paddy-upland rotation system, were clarified in this study. Chemical fertilizer was the main form of P input in Yangtze River Basin, which accounted for 97.2%of total P input in 2010. In this area, phosphorous output was mainly through river export, in which the dominant form was sediment-carried particulate P. Large dams significantly prevented the export of sediment with high P content in the river. Phosphorus balance in Yangtze River Basin was mainly its accumulation. The surplus of P appeared from 1971 and the annual surpluses reached 1.8 million tonnes which was equivalent to one-tenth of the global consumption of P fertilizer in 2010. Compared to the historical changes in other river basins (e.g. the Thames River), P efflux in Yangtze River Basin is lower than its input currently. However, P depletion has a ’delay’ property, thus it is suggested that, there may be a huge risk of P loss in Yangtze River Basin in the future. Therefore, we recommended that:it is urgent to reduce the chemical fertilizer input in the Yangtze River Basin area, which can be achieved by increasing internal organic fertilizer use, improving plants’ efficiency in soil P use and strengthening water conservation and then reducing the loss of particulate P.(2) This study also displayed the quantitative contribution of paddy-upland rotation system to regional food production and yield responses of P fertilizer in Yangtze River Basin. Paddy-upland crop rotation system is the major cropping system in Yangtze River Basin, which produces 80% of total rice production. The changes in cropping system are mainly driven by regional food demand, policy, technology, economy and other related factors. With the increasing fertilizer input and basic crop yield, the increase in rice yield in paddy-upland rotation system decreased from 58.5% in 1950s to 11.7% in 2000s. It is notable that paddy-upland rotation system enhanced rice yield in Yangtze River Basin more than 12 million tons in early period when fertilizer input was limited. The increased rice yield by paddy-upland rotation system in this area accounted for 20% of the total rice production at that time, which was important for ensuring food security in this region. Data analysis showed that P fertilization had less effect on crop production in paddy-upland rotation than in continuous rice or dryland crops. This could be mostly explained by the enhanced soil P availability in the paddy-upland crop rotation system. To optimize this cropping system may help improve P use efficiency and thus benefit the development of sustainable agriculture.(3) This study also clarified the P balance and loss pathways in these three cropping systems, and revealed the mechanism of "high soil P availability and low P accumulation with fertilization" in paddy-upland rotation system in Yangzi river basin. The total biomass and total P content of plants were higher in paddy-upland rotation system than those in continuous rice system or dryland rotation systems, indicating that paddy-upland rotation system has a higher production capacity than the other two. Phosphorus was found to move to deeper soil layers (50-60 cm) with different rates among these three cropping systems. The fastest moving rate was observed in the paddy-upland rotation system, and the surplus of 100 kg P hm-2 enables an increase of 0.95 mg P kg-1 in soil Olsen-P. In contrast, the moving rate in the dryland rotation system was lower and had only 0.48 mg P kg-1, whereas P movement in the continuous rice system was not evident in the study. These results indicated that paddy-upland crop rotation system significantly enhanced P movement in soil profile and improved the P uptake by maize roots in the deeper soil layers. On the other hand, the flooding improved soil reducing capacity and pH, which improved the availability of soil P in the soil surface layer. It is suggested that the "activation of P-transport-absorption-mobilization processes" may be the main mechanisms for P availability in the P cycle of the paddy-upland crop rotation system in Yangtze River Basin.(4) Significantly different rhizosphere processes were found in different upland crop species in terms of foraging P in soil. Legume crops (e.g. Lupinus albus, Cicer arietinum) are mainly dependent on root exudation to mobilize soil P, and the release of root exudates can be induced by low soil P supply. Cereal crops (e.g. lea mays, Triticum aestivum) mainly rely on root proliferation to efficiently acquire soil P, which is sensitive to P deficiency. To our knowledge, this study was the first to use PCA-based comprehensive analysis that integrated with rhizosphere dynamics to demonstrate and quantify the trade-off between root morphological and physiological responses to P acquisition across different upland crop species with contrasting rhizosphere processes. Different from the legumes, cereal crops mainly relied on the changes in root morphology that was sensitive to low P supply to cope with P deficiency. To rescue the low P mobilization capacity by rice relying on root morphological plasticity, the ’drying-wetting cycle’ in the paddy-upland crop rotation systems enhanced the soil P mobilization and improved the P use efficiency for rice. This study provided important knowledge for optimizing the rhizosphere nutrient management to improve P use efficiency for a specific crop species.In conclusion, we clarified the evolution of P balance in Yangtze River Basin, and quantified the historical contribution of paddy-upland rotation system to the food production and fertilizer inputs in Yangtze River Basin. We also discussed the problems in P balance and proposed the possible solutions in this part. The problems in P balance in such cropping system were also discussed and the possible pathways for overcoming the problems were proposed in this study. We further investigated the crop productivity and P balance in three different cropping systems:paddy-upland rotation system, continuous rice system and dryland rotation system. In these experiments, the P movements along soil profiles were monitored. The P use efficiencies in main crops with various rhizophere processes were compared. The results revealed the effects of drying-wetting cropping and plant rhizosphere processes on soil P balance, mobilization and interaction mechanisms between them, which are critical for optimizing P use efficiency in the rhizosphere process-based crop management under a specific cropping system. |
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