| Phosphorus (P) is a major essential element for plant growth, metabolism and productivity, whilstexcess P has resulted in worldwide eutrophication in water bodies. The chemical and biologicalmechanisms of soil P availability are the key points for soil science. The study is mainly to address theevolution characteristics and mechanisms of soil available P under long-term (15–31) fertilizations atvarious eco-climate conditions. Six fertilization treatments were chosen:①Control,②chemical N,③chemical N and P (NP),④chemical NP and K (NPK),⑤chemical NP and manure (NPM);⑥chemical NPK and manure (NPKM) at13long-term experiment sites (four mono-upland sites, fivedouble-upland sites and four rice field-upland sites). Data analyses from these long-term fieldexperiments with a glasshouse pot experiment were used to examine (1) evolution characteristics of soilavailable P (SAP);(2) critical values of soil available P for wheat/barley, maize and rice;(3) response ofsoil available P to P apparent balance under different soils and fertilizations;(4) P cycling insoil-microorganism-plant systems under the addition of organic compounds into soil with differentcarbon/nitrogen ratios. The main findings are as followings:(1) Effects of long-term fertilizations on evolution characteristics of SAP of the13sites weregenerally showed that: SAP was increased by4.2and0.9mg/kg respectively under chemical P plusmanure treatment and chemical P treatments, but decreased by0.2mg/kg under no P treatments. At themono-/double-cropping upland sites, an increase of1.0g/kg soil organic carbon (SOC) would increaseSAP by0.04–0.15mg/kg and P activation coefficient (PAC, percentage of soil available P to total P) by0.3–1.2%, respectively. Though significant relationship showed between SOC and SAP, no significantrelationship showed between SOC and PAC at the upland rice sites.(2) The critical value of SAP for different crops under different soils showed that:7.5–23.5mg/kgfor the wheat/barley,5.7–15.2mg/kg for the maize and4.3–14.9mg/kg for the rice. An increase of2.8–18.9mg/kg SAP based on their initial values would achieve their relevant critical values of SAP forthese sites except Harbin (which was already achieved the critical SAP value based on its initial value).(3) The change in SAP was significantly affected by P apparent balance under different croppingsystems. The mean change in SAP with100kg P/ha surplus ranked as: chemical P plus manuretreatments (11.1mg/kg)> chemical P treatments (2.3mg/kg)> no P treatments (0.7mg/kg) at fivedouble-cropping upland sites, whilst chemical P treatments (mean11.6mg/kg)> chemical P plusmanure treatments (mean6.3mg/kg)> no P treatments (~0mg/kg) at four rice field-upland sites.Change in SAP with P surplus varied among the four mono-upland sites. At mono-/double-croppingupland sites, the promotion of SOC would significantly increase the change in SAP with P surplus.However, there was no significant effect of SOC on the transformation of surplus P into SAP at ricefield-upland sites.(4) Effects of organic compounds (OC) with various C/N ratios on the transformation of P amongsoil-microorganism-plant systems were different under P deficit and P surplus conditions. Under the P surplus conditions, OC with low C/N ratios increased P assimilation by microorganisms and P uptakeby plants from soil available P fractionations. The microbial biomass P fractionations were5.4%,4.7%and1.7%, whilst plant P fractionations were14.8%,9.5%and2.1%, respectively. However, OC withlow C/N ratios only increased P assimilation by microorganisms, but had less impact on the P uptake byplants.In summary, the use of organic manure to increase soil available P is more effective in Chinesetypical upland (especially double-cropping upland) soils than in rice-field upland soils. In agriculturalpractices, adapting organic manure with lower C/N is more reliable to promote P uptake by crops and Passimilation by microorganisms under P surplus conditions. |
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