Effects Of Long-term Fertilization On Soil Phosphorus Fractions And ¹³C And ¹⁵N Natural Abundance In Lei Bamboo ?Phyllostachys Praecox? Forest And Paddy Fields

Fertilizer application is an important agricultural practice to improve soil fertility and promote crop yield.However,excessive fertilizer applications have resulted in increased acidification and other soil degradation issues.Carbon,nitrogen and phosphorus are three of the indispensable elements for plant growth,and the loss of soil nitrogen and phosphorus caused by excessive fertilization is the main reason that worsens the eutrophication of waterbody.In this study,the effects of long-term fertilizer application on soil phosphorus fractions and the ¹³C and ¹⁵N natural abundance in Lei Bamboo?Phyllostachys praecox?stands and paddy soils were investigated.The soil physicochemical properties such as pH and soil organic carbon?SOC?were determined.The concentrations of different P fractions were measured with modified Hedley's sequential extraction procedures.The natural abundance of¹³C and ¹⁵N(?¹³C and?¹⁵N)in soils were determined by Isotope Ratio Mass Spectrometer?IRMS?.The main results are as follows.?1?To investigate the characteristics of soil phosphorus fractions and the ¹³C and ¹⁵N isotopic signatures in Phyllostachys praecox stands,samples of upper?0-20cm?and lower?20-40 cm?soil layers were collected from stands which were intensively managed for 3,9 and 15 years,respectively.The results showed that the pH of top soil significantly decreased in the 9-and 15-year intensive management plots where acidification of subsurface soil was observed.The nitrogen and SOC mainly accumulated in the upper soil layer,and the highest TN and SOC contents were observed in the 15-year intensive management plots,which increased by 92%and 183%,respectively,compared to the 3-year intensive management plots.Moreover,theconcentrationsofTP,OlsenP,labileP?H₂O-P+NaHCO₃-Pi+NaHCO₃-Po?and moderately labile P?NaOH-Pi+NaOH-Po?in both soil layers all increased with the prolonging of plantation time.Compared with the 3-year intensive management plots,the proportion of labile P and moderately labile P in the 15-year intensive management plots increased by 63%and 83%for the upper soil layer,and 88%and 128%for the lower soil layer,respectively;the proportions of sparingly labile P?HCl-P+Res-P?in soil decreased from 58%to 26%in the upper layer and from 66%to 29%in the lower layer,respectively.The concentrations of sparingly labile P in soil were not significantly affected by the number of years of intensive management.The ?¹³C in both soil layers reduced with the increase of years of intensive management,and the reduction in the upper soil layer was greater than that in lower layer soil.Whereas the ?¹⁵N in the upper soil layers was greater than that in the lower layer soil.The highest ?¹⁵N was found in the upper soil layer in the 9-year intensive management plots,and the ?¹⁵N in the lower soil layers decreased with the number of years of intensive management.Therefore,it is important to reduce amount of chemical fertilizers in Phyllostachys praecox stands in order to minimize the risk of soil N and P loss.?2?In order to study the effects of different fertilization on phosphorus fractions and the ¹³C and ¹⁵N natural abundance in the paddy soil,samples were collected from a long-term fertilizer field trial,established in Hangzhou,Zhejiang Province,China,in 1996.Six treatments including no fertilizer application?Control?,combination of chemical fertilizers and rice straw?NPKRS?,combination of chemical fertilizers and organic manure?NPKOM?,chemical fertilizers only?NPK?,rice straw only?RS?,organic manure only?OM?were designed in the experiment.Each treatment had three replicates.The topsoil?0-20 cm?samples were collected from each plot on the trial site in November 2012.The results demonstrated that application of the organic manure alone or combined with chemical fertilizers significantly increased the concentrations of SOC,TN,TP and Olsen P,and the largest accumulations were all found in the NPKOM treatment.According to the results of P fractionation,the HCl-P?40.9%⁵0.0%of the total P?was the predominant inorganic P fraction in the paddy soil,whereas the main organic P form was NaOH-Po?6.1%⁹.7%of the total P?.Labile P?NaHCO₃-Pi+NaHCO₃-Po??2.8%⁸.4%of the total P?constituted the least soil P fractions,indicating a low P availability in this paddy soil.Proportion of the Res-P in the NPKOM treatment was 10.5%lower than that in the Control,whereas the proportion of the NaHCO₃ extractable labile P in the NPKOM treatment was 5.6%higher than that in the Control.Moreover,the NPKOM treatment had the lowest soil ?¹³C and the highest soil ?¹⁵N,which indicated that application of chemical fertilizers combined with organic manure was an effective way to promote the accumulation of new soil organic matter,improve soil fertility and reduce soil nitrogen loss thereby reducing the risk of eutrophication of water bodies.In conclusion,the long-term application of chemical fertilizer caused the accumulation of soil P and acidification in both Lei Bamboo?Phyllostachys praecox?forest and paddy soils.The applications of chemical fertilizers and organic manure promoted the accumulation of SOC and resulted in the decrease of ?¹³C in soils under both land uses.In addition,long-term application of nitrogen fertilizer could increase the TN content and ?¹⁵N in both Lei Bamboo forest and paddy soils.