Fate of low-molecular-weight organic phosphorus compounds in the P-rich and P-poor paddy soils

Continuous application of organic fertilizers can cause accumulation of organic phosphorus(P)in soil,especially in the lowmolecular-weight organic phosphorus(LMWOP)forms.This organic P pool represents a potentially important source of P for both plants and microorganisms.To understand the effect of long-term fertilization(30 years)(P-rich soil)vs.fallowing(P-poor soil)on the bioavailability and fate of LMWOP in subtropical paddy soils,we determined the sorption and mineralization of 14 C-labeled adenosine,adenosine monophosphate(AMP),adenosine diphosphate(ADP),and adenosine triphosphate(ATP)in each soil.The contents of carbon,nitrogen,and P in the P-rich soil were more than two times greater than those in the P-poor soil.The mineralization rates of the LMWOP compounds were faster in the P-rich soil compared to the P-poor soil,and followed the order AMP>ADP>ATP.Using sterilized soil,all forms of adenosine-P were strongly sorbed to the solid phase and reached saturation in a short time,with the adsorbance increasing with the number of phosphate groups.We concluded that the mineralization of LMWOP compounds was repressed slightly by sorption to the solid phase,but only in the short term.Thus,LMWOP compounds serve as readily available sources of C for microorganisms,making P available for themselves as well as for the plants.However,P accumulation and the progressive saturation of the P sorption sites in highly fertile soils may increase the potential risk of P runoff.

Effect of N and P fertilization on the allocation and fixation of photosynthesized carbon in paddy soil

Potted rice seedlings independently treated with N,P,and NP were continuously^(13)CO_(2) labeled to investigated the influence of N and P application on the contribution of photosynthesized C to the rhizosphere versus bulk soil and particulate organic matter(POM)versus mineral fraction(MIN).N and NP enhanced net assimilated^(13)C on day 14(D14),with maximum C assimilation occurring on day 22(D22)under NP.Aboveground biomass retained more^(13)C than belowground biomass for all treatments.^(13)C incorporation into the rhizosphere exceeded that in bulk soil,with the maximum(6-10%)found under N addition.Newly assimilated^(13)incorporated into POM increased in the rhizosphere under N and NP conditions,whereas MIN remained largely unaffected.^(13)C-MBC proportion in the total microbial biomass C(MBC)pool revealed that N and NP stimulated microbial activity to a greater degree than P.The main portion of^(13)C in the rhizosphere and bulk soil was found in POM on D14,which decreased over time due to microbial utilization.Contrastingly,root-derived ^(13)C in the MIN remained unchanged between sampling days,which indicates that the stabilization of rhizodeposits in this fraction might be the potential mechanism underlying SOM sequestration in paddy soils.

Experimental strategies to measure the microbial uptake and mineralization kinetics of dissolved organic carbon in soil

Soil organic matter turnover rates are typically estimated from mass loss of the material over time or from on rates of carbon dioxide production.In the study,we investigated a new way to characterize the concentration-dependent kinetics of amino acids used by measuring microbial uptake and mineralization of ^(14)C-alanine.We measured the depletion from soil solution after additions ^(14)C-alanine.The microbial uptake of ^(14)C-alanine from soil solution was concentration-dependent and kinetic analysis indicated the operation of at least three distinct alanine transport systems of differing affinities.Most of the ^(14)C-alanine depletion from the soil solution occurred rapidly within the first 10-30 min of the incubation after 10μM to 1 mM substrate additions.At alanine concentrations less than 250μM,the kinetic parameters for K_(m) and V_(max) of the higher-affinity transporter were 60.0μM and 1.32μmol g^(-1) DW soil h^(-1),respectively.The mineralization of alanine was determined and the half-time values for the rapid mineralization process were 45 min to 1.5 h after the addition at alanine concentrations below 1 mM.The time delay after its uptake into microbial biomass suggested that alanine uptake and subsequent respiration were uncoupled pattern.The microbial N uptake rate was calculated by microbial mineralization,and an estimated K_(m) value of 1731.7±274.6μM and V_(max )value of 486.0±38.5μmol kg^(-1)DW soil h^(-1).This study provides an alternative approach for measuring the rate of turnover of compounds that turnover very rapidly in soil.