Effects Of Maize/Alfalfa Intercropping And Phosphorus Application On Soil Inorganic And Organic Phosphorus Fractions

Phosphorus is a crucial nutrient for crop growth,but most agricultural soils lack sufficient available phosphorus resources to meet crops need.Applying phosphorus fertilizer can increase the active phosphorus fractions in the soil and ensure crop growth.However,most of the applied phosphorus fertilizer is converted into fractions that are difficult for crops to use,making it urgent to find ways to improve uptake and utilization of these fractions.Intercropping can enhance crops uptake of various phosphorus fractions by regulating inter-root processes,but research on the interaction between intercropping and phosphorus application on soil inorganic and organic phosphorus fractions is still limited.Therefore,it is essential to investigate the effects of maize（Zea mays L.）and alfalfa（Medicago sativa L.）intercropping and phosphorus application on soil inorganic and organic phosphorus fractions.The field experiment consisted of two phosphorus application levels,P0（without phosphorus fertilizer application）or P1(52 kg P hm^-2for maize and 59 kg P hm^-2for alfalfa),and three cropping patterns,MM（monoculture maize）,MA（monoculture alfalfa）and IMA43（4 rows of maize with 6 rows of alfalfa）,with four replicates.The soil inorganic phosphorus fractions were extracted according to the sequential fractionation scheme modified by Jiang and Gu,and detailed classification includes dicalcium phosphate（Ca₂-P）,octacalcium phosphate（Ca₈-P）,variscite（Al-P）,strengite（Fe-P）,hydroxylapatite(Ca₁₀-P)and occluded P（O-P）.The soil organic phosphorus fractions were extracted according to the sequential fractionation scheme proposed by Bowman and Cole,and detailed classification includes labile organic phosphorus（LOP）,moderately labile organic phosphorus（MLOP）,moderately stable organic phosphorus（MSOP）,highly stable organic phosphorus（HSOP）.The main findings based on five years of field trial data are as follows:（1）When no phosphorus fertilizer was applied,intercropping reduced the proportion of stable inorganic phosphorus to total inorganic phosphorus and had a significant effect on the concentration of LOP and MSOP.In the later stages of the experiment,the percentage of stable inorganic phosphorus was reduced by 3-5%in IMA43 compared to MM and MA.The concentrations of LOP and MSOP in IMA43 soils were generally lower than those in MM and MA,especially the LOP in the 0-10 cm soil layer in 2016 and the MSOP in 0-10 cm and10-20 cm soils in 2018 were the most significant differences,with significant reductions of45.9%,29.0%and 32.3%,respectively,compared to MM.（2）The application of phosphorus fertilizer increased various soil inorganic and organic phosphorus fractions to varying degrees.The applied phosphorus fertilizer was mainly fixed in the form of inorganic phosphorus,and O-P accumulated earlier in 2015 when phosphorus was applied compared to no phosphorus application.Among the organic phosphorus fractions,phosphorus application had the greatest effect on LOP and also promoted the accumulation of organic phosphorus fractions that are difficult to utilize and transform.（3）The interaction of phosphorus application and intercropping had significant effects on Ca₂-P,Ca₈-P,HSOP in the 0-10 cm soil depth and HSOP in the 10-20 cm soil layer.The concentrations of Ca₂-P,Ca₈-P and Ca₁₀-P in IMA43 soils were lower than MM in the 0-10cm soil layer from 2015-2018,with the most significant difference in Ca₂-P,which was reduced by 38.5-67.1%compared to MM,followed by Ca₈-P,which was reduced by10.9-27.2%.In the 10-20 cm soil layer in 2016,Ca₈-P in IMA43 was significantly increased by 44.6%compared to MM.In 2014,HSOP of IMA43 decreased by 19.3%compared to MM in the 0-10 cm soil layer and significantly decreased by 22.5%in the 10-20 cm soil layer.The HSOP of IMA43 was significantly decreased by 56.8%compared to MM in 10-20 cm soil layer in 2015.（4）Significant correlations were found between various inorganic and organic phosphorus fractions of soils and soil pH,soil moisture content,plant phosphorus uptake,and total crop yield.Whether phosphorus was applied or not,soil pH was significantly negatively correlated with both O-P and MSOP,soil moisture content was significantly positively correlated with O-P and MSOP,and plant phosphorus uptake was significantly positively correlated with moderate-cycling inorganic phosphorus fractions.In summary,maize/alfalfa intercropping can decrease the proportion of stable inorganic phosphorus and MSOP content under no phosphorus application conditions.Under phosphorus application conditions,intercropping can facilitate crops utilization of Ca-P,which is abundant in alkaline soils,and slow down the accumulation of HSOP.Therefore,establishing a maize/alfalfa intercropping system can fully utilize soil phosphorus resources in alkaline soils with low phosphorus,ensuring food supply and sustainable livestock development.These findings provide a theoretical basis for exploring the effects of intercropping patterns and phosphorus application on soil inorganic and organic phosphorus fractions.