Phosphate adsorption on paddy soil particles affected by application of Chinese milk vetch

Objectives Studying the effect of Chinese milk vetch (CMV) on phosphate (P) adsorption on soil particles could provide a theoretical basis for efficient green manure usage.

Methods Paddy soil was collected from Anhui Province, and CMV was added at the rate of 0, 15000, 22500 and 30000 kg/hm², and denoted as CMV0, CMV1, CMV2, and CMV3. The mixture was incubated for 30 days under anaerobic conditions. Each soil sample was divided into sand (48–250 μm), silt (2–48 μm), and clay (<2 μm) particles for the isothermal and kinetics adsorption experiment.

Results Compared with CMV0, CMV (P<0.05) increased the content of organic matter (SOM), total nitrogen (TN), total phosphorus (TP) and available phosphorus (AP) in all the three soil particle sizes. The highest increase of 33.42%–81.04%, 4.83%–15.17%, 45.45%–51.52%, and 40.76%–60.70% was recorded in sand particles. CMV reduced the specific surface area of sandy and clay particles but increased those of silt particles. The Langmuir model described well the P adsorption of soil particles. The maximum adsorption capacity (Q_m), native adsorbed exchangeable phosphorus (NAP), zero-equilibrium P concentration values (EPC₀), adsorption constant (K_L), and soil affinity to phosphorus (K_p) in all soil particles increased under CMV. Sandy particles recorded the highest increase, with the Q_m reaching 4.02%–46.81%. In silt particles, the NAP, K_L, EPC₀, and K_p increased by 116.77%–210.78%, 29.55%–69.05%, 93.62%–141.28%, and 11.97%–28.87%, respectively. The kinetic P adsorption was well fitted with the Pseudo-second-order kinetics. The fitting results showed that the adsorption rate (k₂) and the initial sorption rate (H) of the different soil particles increased with CMV application. The highest H and k₂ increases were recorded for clay particles, increasing by 25.77%–98.20% and 25.74%–111.15%, respectively. Notably, CMV2 recorded higher H and k₂ values than CMV1 and CMV3 in all soil particles. The Q_m and EPC₀ were correlated with AP in sandy particles (P<0.01) . The NAP, EPC₀, and K_p were correlated with SOM and TP (P<0.05) ; K_L was correlated with TP in silt particles (P<0.01) . In clay particles, Q_m was correlated with TP and AP (P<0.01) ; EPC₀ was correlated with SOM, TN, TP, and SSA; K_p was correlate with SOM, TN, and SSA (P<0.05) .

Conclusions The adsorption of phosphate on the soil particles was enhanced by improving TP and AP mainly with the application of CMV, especially on the sandy and silt particles. The highest values of Q_m and K_L in CMV2 were recorded for sandy and clay particles. In CMV3, the silt particles had the highest Q_m and K_L. Therefore, rational application of CMV to paddy soil combined with soil texture could help achieve scientific management of P.