| Rice is non-arguably the most important staple crop in the world.Paddy soil is an anthropogenic soil,serving as an important soil resource in China.In essence soils poss charges on their surface,is their intrinsic complexity and heterogeneity on the surface reactions.Generally,soil colloids are predominantly made up of clay minerals,Fe/Al(hydrous)oxides,organic matters and microorganisms.They will change largely under long-term paddy cultivation when compared with those in the corresponding upland soils.The surface charge on soil is mainly held by the colloids(<2μm).Thus,the surface electro-chemical properties of paddy soils,and further the chemical behaviors of ions on these soils will vary with the upland soils.A chronosequence of paddy soil derived from Afisol located in a temperate region of Northeast China and a paddy soil derived from Oxisol in subtropical region of Southern China were studied in this paper,respectively.The evolution characteristics of soil surface charge along with the Alfisol-derived paddy soil chronosequence were investigated.The phosphorus immobilization and involved mechanisms by the paddy soils,and potential phosphorus loss risk on these soils were assessed.The surface properties of Oxisol-derived paddy soil were evaluated,as well as the phosphate adsorption behaviors and the influenced factors.These results help to improve our understanding of soil surface electro-chemical properties along with paddy soil chronosequence,and provide theoretical guidance for the paddy soil cultivation scheme,and further plan remediation strategies.Our results showed:(1)Intermittent flooding significantly changed the surface electro-chemical properties of paddy soils when compared with their upland soil.Soil surface charge increased with rice cultivation age,which was mainly attributed to the increase of organic matter and clay fractions.The contents of organic matter significantly increased with paddy cultivation age,resulted in an obvious increase in the variable surface charge.Meanwhile,hydroponics cultivation prompts the destruction and weathering of primary minerals and the formation of soil phyllosilicate minerals,which were mainly responsible for the increase of permanent surface charge on the Alfisol derived paddy soils.Thus,it is anticipated that an increasing nutrient retention capacities of Alfisol derived paddy soils with the paddy soil chronosequence over a century scale,owing to an increasing soil CEC.While the decreasing soil pH would alleviate the efficacy.The contents of Fe/Al oxides increased observably with increasing duration of rice cultivation,and reached a plateau in the 30-yrs paddy soil.The contents of exchangeable calcium in paddy soils were higher than that in upland soil except for the 8-yrs paddy soil,owing to the heavy inputs of calcium-containing phosphate fertilizer and weathering of minerals.(2)P adsorption isotherms suggested that P adsorption capacities increased with rice cultivation age,and reached a plateau at 30-yrs paddy soil.The capacities were closely correlated with the contents of Fe/Al oxides and exchangeable calcium(P<0.05)in the paddy soils.Furthermore,the sequential extraction of pre-adsorbed P revealed that phosphate preferentially binded by calcium carbonate,followed by Al oxides and then Fe oxides.P desorption results indicated that about 18.7-19.7%of pre-adsorbed P can be desorbed when 100 mg P L-1 were added.Most adsorbed P were retained on the paddy soils.In general,the contents of Olsen-P in the studied paddy soils were higher than 20 mg kg-1,and DPS values were below 10%,indicated that there are enough available P for plant growth and the P loss risk to nearby waters is low.Therefore,rice planting is a sustainable and environment-friend cultivation strategy.(3)Intermittent flooding leads to a significant decrease in free Fe oxides and an increase in amorphous Fe oxides in Oxisol-derived paddy soil,and rice cultivation diminished P adsorption.Long-term paddy cultivation led to an increase in P desorption,DPS,soil pH,and a progressive loss of fine particles,which may in turn lead to an increase in environmental risk of P loss in paddy soil,when compared with the upland Oxisol. |
PDF Full Text Request