| Low phosphorus (P) use efficiency is a great challenge for implementation of the chemical fertilizer’s "Zero Increase Action Plan", and it is also one of the major limiting factors for sustainbale agriculture production. Under P deficiency, plants have displayed an array of adaptation characteristics for P acquisition to different soil P supply, such as changed root morphology (spatial factors) and root physiology (rhizosphere chemical processes). However, the related prcessess and approaches of manipulating root growth by regulating soil P supply intensity and root growth volume for enhancing P acquisition were not clear in Chinese major croplands with high soil P fertility. Revealing the pattern of root responses to soil P supply intensity and root growth volume is very important to optimaize P management and hence to improve P use efficiency in P accumulated soils. In the present study, based on a consecutive 6-year P fertilizer field experiment conducted in Quzhou county and the previous published data about yield respnses to P fertilization in China, the pattern of root responses to long-term P fertilization, the rhizosphere processes related to high residual P use and the potential for reduced P input in China’s major croplands were evaluated. Root growth and P uptake in response to soil P supply intensiy and root growth volume were examined. Further, P acquisition by maize under different soil water supply levels was assessed and the potential and effect of combination of inorganic and organic fertilizer (Pi and Po) application on maize growth and yield was evaluated. Key points of the results and progress were summaried as follows:1. The size of soil residual P pool in China’s major croplands and the magnitude of yield increase due to P fertilization were quantified. Huge amount of P was accumulated in soil due to excessive P fertilization, improving inherent soil P availability. Crop yield in response to P fertilization in different historical periods was significantly affected by soil P supply intensity affected by consecutive P input in three major cropping systems (W-M:wheat-maize, W-R: wheat-rice and R-R: rice-rice). The percentage of yield increase due to P fertilization increased firstly and then declined in W-M and W-R cropping systems since 1980s. The magnitude of yield increase was kept about 10%. Yield increase response to P fertilization was negilible in the last decade. These results indicated that there was a greater potential for reduing P application but keeping high P use efficiency in the major three cropping systems.2. The study clarified maize root responses to soil P supply intensity and soil P use efficiency by roots, revealing the mechanism of P use by maize under intensive farming system. The use of soil residual P by maize was examined and the potential approach to improving soil P use in calcareous soil was evaluated. Maize yield did not decrease in the P-omision treatment in the first three years (2010-2012) since the experiment started, indicating that soil residual P could meet the demand of maize for three years in the calcareous soil. Maize yield and the amount of P exploited from the soil by maize in the treatment P2 (75 kg P2O5 ha-1) increased with time after 2010. The root exploitation scope and root efficiency in P75 were significantly higher than those in P0. These results suggested that soil P could be efficiently used by modifing root growth through maintaining the appropriate soil P supply intensity (12.5 mg kg-1) in the root zone.3. The pattern of maize responses to soil P supply intensity and root growth volume was evaluated. The tradeoff relationship between soil P supply intensity and root growth volume was observed. At the same root growth volume, P uptake by maize could be enhanced by modifing root growth through improving soil P supply intensity, whereas, under a certain P supply level, maize P uptake could be also improved by extending root growth volume. The results indicated that a certain growth volume could be necessary for high P acquisition by maize roots.4. The processes and magnitude of maize responses to soil water and soil P supply intensity were analyzed. Under medium soil moisture stresses, maize P uptake could be improved by modifying root growth through enhancing soil P supply intensity. P uptake by maize could also be enhanced by stimulatingt root exploitation scope through optimizing soil moisture condition under P deficiency. The results indicated that it is a potential and effective approach to manipulate soil water availability to modify root growth and root exploitation scope to enhance soil P acquisition in the calcareous soil.5. The potential and effect of combination of inorganic and organic fertilizer (Pi and Po) application on mazie growth and grain production were assessed. The combination of inorganic and organic P fertilizer could reduce chemical P input by 50%, indicating an evident replacement effect of Pi by Po. Root growth could be modified by organic P (manure) as well as chemical P fertilizer to improve P uptake by maize. The N-based organic feretilizer application resulted in the surplus of P in maize cropping system, but the P-based organic feretilizer application sustained the balance of P in maize cropping system (but need extra N addition). The latter could be recommended for organic fertilizer management in maize production. The results suggested that organic manure as organic P could be a potenatial alternative replacement for chemical P fertilizer, and P-based manure management needs to be emphasized for organic fertilizer application in high-yield maize production.To sum up, in the present study with meta analysis based on publicated literatures and a P fertilizer long term field experiment, the mechanism and potential approaches to efficiently utilize soil P by maize in P accumulated soil were evalued by quantifying the size of soil residual P pool in China’s major croplands and yield increase responses to P fertilization. The P long term experimental results showed that soil P accumulated could maintain the same grain yield for 3 years without P input, indicating a greater level of soil P supply. There was an evident trade off between soil P supply intensity and root growth volume for efficient P use by maize. The appropriate soil water supply enhanced root growth and spatial availability of soil P, resulting in increased P use efficiency. The 50% of total P input could be replaced by organic P fertilizer as organic P resource, and the combination of organic fertilizer and chemical fertilizer could maintain the relativity stable maize yield. The optimized P management strategies were proposed to increase P use efficiency and sustain maize growth and grain yield, involving keep a proper soil P supply intensity and soil moisture status, coordinating soil P supply level and root growth volume, and combination of organic and inorganic fertilizer to partially replace chemical P fertilizer input, which was regarded as an effective approach for increasing P use efficiency in the intensive maize production system. |
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