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It is of paramount importance to assess phosphorus (P) accumulation and distribution in deep soil profile after land use changes to improve current P management. This study investigated soil P fractions and degrees of P saturation in 300-cm soil profile after conversion of wheat-maize cropping to greenhouse vegetable or kiwi planting in Yangling, Northwest China. Compared with the arable land, vegetable greenhouse or kiwi orchard showed significantly greater soil P accumulation at surface layer, i.e., CaCl 2-P increased by 10-20 times, Olsen-P by 3-5 times, Mehlich-3-extracted P by 3-6 times, and organic-P by maximum of 4 times. A degree of P saturation increased from 4 under arable crop to 14% under kiwi orchard and 22% under greenhouse vegetable. Land conversion also markedly increased soil available P (Olsen-P and Mehlich-3-extracted P) distributing in deep soil layers down to 300 cm, especially for greenhouse vegetable, where Olsen-P or Mehlich-3-extracted P contents reached the critical P value for arable crop to achieve maximum yield. Our results indicate a huge amount of P had been further leached down to deep soils under the current management. Hence, there is an urgent need to update the P management strategies under greenhouse vegetable and kiwi orchard, especially for those involving in amendment of organic manure, to prevent the situation from getting worse in future.

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Seeking food security, contemporary Chinese agriculture has followed a trajectory of overfertilization and associated environmental problems, hence the need for nitrogen-balancing practices that do not compromise yield and quality. Here we present a national meta-analysis using 224 studies with 1972 comparisons to quantify the potential to reduce nitrogen (N) fertilization to improve environmental outcomes whilst maintaining yield and grain protein. We calculated a nitrogen reduction ratio NRR, as 100 × (NC - NT) / NC; where N is N fertilizer rate and subscripts indicate farmer practice (C) and reduced N rate treatment (T). Our meta-analysis showed that the NRR that maintained yield and grain protein content at the level of current practice was up to 10% in wheat and up to 30% in maize and rice. Larger yield-neutral NRR could be achieved in more fertile, heavier-textured soils, and with practices including enhanced-efficiency N fertilizer, combined application of organic and inorganic N fertilizer, and incorporated straw. Assuming a reduction in N fertilizer usage by 10% for wheat and by 30% for maize and rice in the current cropping area, there is a potential to save 5.7 Mt N yr-1; reduce loss of reactive nitrogen by 1.26 Mt N yr-1, equivalent to 63% of annual total Nr losses for rice in China, reduce N-related greenhouse emissions by 75.2 Mt CO2-eq yr-1, equivalent to 14.5-25% of the emissions associated with the N fertilizer chain in China; and improve N use efficiency by 23%. Our results highlight the feasibility of maintaining yield and grain protein, and achieving substantial environmental benefits with reduced fertilization rate, and the environmental and agronomic scenarios where these outcomes are more likely.