Effects Of Plant Materials And Water Management On PH And Carbon/Nitrogen Mineralization In Paddy Soils

A Psammaquent and a Plinthudult soil were collected from Longyou County and a Paleudalfs soil was collected from Jiaxing in Zhejiang Province, in the south of China. Chinese milk vetch, wheat straw and rice straw were used in this study. These plant materials were collected from mature plants in the field. Air-dried soils added with plant residues were incubated under the laboratory conditions. Three water treatments were used:control water (CW), wetting-drying cycles (W-D), and water submergence (WS). By measuring the soil pH, NH4+, NO3-, CO2, MBC, DOM, DON and other indicators, the effects of plant residues and water regimes on soil acidification were characterized and the mechanisms of the soil acidification and other aspects were systematically explored and discussed. The main results are as follows:(1)Addtion of plant straws significantly increased pH of acidic paddy soils with the different initial pH during the initial three days. After3days, soil pH increased in Psammaquent and Plinthudult soils and decreased in Paleudalfs as incubation time increased. The magnitude of soil pH change was less in the soil with higher buffer capacity. The effect of plant residues on soil pH change was longer and stronger in soils with lower initial soil pH than with higher initial soil pH. Alkalinity production was affected by the type of the residues, being higher in Chinese milk vetch than in wheat straw and rice straw. Soil pH increase was positively correlated with alkalinity and negatively correlated with C/N ratio of the residues.(2)Net pH increase was found in two soils upon addition of CM vetch under WS. It was found that the net increase of soil pH relative to its control soil was much higher inPlinthudult soil after addition of CM vetch under WS, and the effect of W-D on net pH increase in Paleudalf soil was similar to WS treatment after addition of CM vetch. Under W-D, soil pH change with incubation time was relatively small by addition of CM vetch in Paleudalf soil. It was observed that the net increase of soil alkalinity after addition of CM vetch under W-D relative to its control soil was much higher and the effect was much longer in Paleudalf soil. Thus, W-D should also be used for managing pH. It was observed that the net increase of soil alkalinity relative to its control soil was much higher in Plinthudult soil after addition of CM vetch under WS. Alkalinity had significant positive correlation with pH changes (p<0.01).(3)Soil microbial biomass carbon (MBC) dramatically increased following by incorporation of the plant materials. The residue type had a significant effect on MBC with the order of CM vetch> rice straw> wheat straw. It increased with incubation time, reached the maximum at day3, then decreased and became relatively stable from day14. On average, highest MBC was observed in Paleudalfs soil while lowest was observed in Psammaquent soil. MBC was significantly positively correlated with organic matter content, and the soil microbial biomass C/N was significantly correlated with soil pH changes (R2=0.87, p<0.01). In this experiment, the C/N ratio of CM vetch, rice straw, and wheat straw was16.8,39.9, and75.3, respectively. The addition of CM vetch resulted in net N mineralization while addition of wheat and rice straws resulted in net immobilization. Which was related to the C/N ratio of plant materials. Obviously, microbes played main roles in mineralization and mediated alkalinity production after incorporation of the plant materials, and soil pH was thereby increased. The addition of CM vetch restrained the activity of the microbes in W-D and WS. However, the addition of CM vetch dramatically increased MBC in all the water treatments, the increase being greatest under CW while smallest under WS. Amount of MBC concentrations were initially increased, reached the maximum, and then decreased. The amount of MBC concentrations was always higher in Paleudalfs than in Plinthudult under the water treatments. At the end of incubation, the decrease of MBC was the highest in CW, and MBC content was higher in W-D than in WS.(4)Addition of plant straws increased the accumulative CO2emission. Soil respiration rate rapidly increased. It was usually highest at the third day, and then decreased with time of incubation. Respiration rate was highest in Paleudalfs and lowest in Psammaquent. Addition of plant straws increased soil respiration and thereby increased the emission of CO2.The order of the effect of plant residues on soil respiration rate followed:Chines Milk vetch (CM vetch)> Rice straw(RSt)> Wheat straw(WSt). From the first day to the third day, DOC concentration rapidly decreased, and the organic anions in the DOC made a contribution to the increase of soil pH especialy at the initial age. DON concentration was positively correlated with N concentration and excess cations, and was negatively correlated with C/N. In Psammaquent and Plinthudult soils, with addition of CM vetch, NH4+concentration increased with time of incubation. CM vetch promoted NH4-N release and increased NO3-N concentration. Rice and wheat straws promoted NH4-N fixation and decreased NO3-N concentration. Results indicated that the NH4+concentration was highest in soils with addition of CM vetch, the order of NH4+concentration was:Psammaquent> Plinthudult> Paleudalfs.(5) In CW, DOC concentrations decreased with time of incubation in two soils with CM vetch added. WS and W-D were better than CW in terms of slowing down the decrease of DOC concentrations. DOC concentrations were much higher in less acidic soils rich in basic organic matter than in more acidic soils of lower basic organic matter in WS and W-D. After addition of CM vetch in the two soils, the respiration rate and cumulative CO2were higher in the soil of rich in basic organic mater in CW and WS (p<0.01). The amounts of cumulative CO2slowly increased in WS than in CW, indicating that the WS condition limited microbial decomposition.(6)With CM vetch added and under W-D and WS conditions, DON concentrations were always higher in Paleudalfs than in Plinthudult soil (p<0.01). In more acidic soils with lower basic N, DON concentrations were highest in CW. In less acidic soils with higher basic N, DON concentrations were higher in WS than in C W. The DON concentration increased in a fluctuating pattern in W-D and it was higher in W-D than in CW. For different water treatments and soil types added with CM vetch, the NO3-N concentrations followed the order of CW> W-D> WS. Thus, pH in Plinthudult soil increased less in CW than in W-D and WS. For the same reasons, NO3-N had a great contribution to soil pH decrease in Paleudalfs under CW. In W-D, frequent wetting-drying cycles could increase N mineralization in acidic paddy soils. With CM vetch added, NH4-N concentration was higher in Plinthudult than in Paleudalfs soil, and NH4-N increase was higher in more acidic soil with lower basic N in WS while the NH4-N increase was lower in less acidic soil with higher basic N in WS.