Continuous application of organic fertilizers can cause accumulation of organic phosphorus (P) in soil, especially in the low-molecular-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 ¹⁴C-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. 

The application of straw and biochar is widely practiced for the improvement of soil fertility.  However, its impact on microbial functional profiles, particularly with regard to paddy soils, is not well understood.  The aim of this study was to investigate the diversity of microbial carbon use patterns in paddy soils amended with straw or straw-derived biochar in a 3-year field experiment in fallow soil and at various development stages of a rice crop (i.e., tillering and blooming).  We applied the community level physiological profiling approach, with 15 substrates (sugars, carboxylic and amino acids, and phenolic acid).  In general, straw application resulted in the greatest microbial functional diversity owing to the greater number of  available C sources than in control or biochar plots.  Biochar amendment promoted the use of α-ketoglutaric acid, the mineralization of which was higher than that of any other substrate.  Principal component analyses indicated that microbial functional diversity in the biochar-amended soil was separated from those of the straw-amended and control soils.  Redundancy analyses revealed that soil organic carbon content was the most important factor regulating the pattern of microbial carbon utilization.  Rhizodeposition and nutrient uptake by rice plants modulated microbial functions in paddy soils and stimulated the microbial use of N-rich substances, such as amino acids.  Thus, our results demonstrated that the functional diversity of microorganisms in organic amended paddy soils is affected by both physicochemical properties of amendment and plant growth stage.