Changes In Soil Properties And Its Relation With Soil Productivity Under Long-Term Fertlization In Paddy Fields

Long-term application of fertilizers not only influence on rice yield and its various trend, but also lead to change in soil properties. However, by far there are few reports that fertilization result in the change of soil properties, which consequently influences soil nutrients capacity and soil productivity. In the present research, a 25 years fertilization experiment was performed in paddy soil. The objectives of studies are 1) to study on change in the paddy soil properties and its effect on conserved ability of nutrients; 2) to investigate the influence of wetting and drying cycle on soil organic matter mineralization in the presence of soil microbe; 3) to analyze the variance characteristics of paddy soil productivity and the factors to paddy soil productivity change under long-term chemical fertilizer application. The experiment consist of different sets of N, P and K (N,P,K,NP,NK,PK), two leave of NPK(NPK and 2NPK) and chemical-organic combined treatment (NPKOM). No application of fertilizer is considered at control treatment. Main results of present research are showed in the next section.1) Change in soil physical properties and its effect on soil nutrients adsorption: Compared with initial sample, soil organic carbon content with treatments was increased to some extent under long-term fertilization except for N treatment. NPKOM treatment had the highest concentration of soil organic matter, and followed by NK, 2NPK and NP treatments; the others were the lowest. Total content of N, P and K in soil with all treatments enhanced in some scale, and the biggest increasing extent occurred in NPKOM treatment among them. In comparison with control treatment, the concentration of soil microbial biomass carbon and nitrogen augmented with most treatment besides NPK treatment. NPKOM treatment had the highest content of soil microbial biomass C and N among all treatment and was increased by 46% of that with control treatment. There was significantly positive correlation between soil microbial biomass C content and soil respiration intensity (R=0.929*). Soil clay content with all treatments was decreased by 3.5%-4.5% after long-term cultivation in comparison with original sample, and varied from 19.3% to 20.5%. There was not marked difference among treatments as for soil clay content. Long-term application of K fertilizer and organic manure enhanced soil aggregate stability (mean weight diameter), but there was slight influence in the others treatments.The concentration of vermiculite-chlorite increased, and mixed layer mineral decreased, whereas content of kaolinite and illite did not markedly change in control treatment. However, in treatments applied N or K fertilizers, the content of vermiculite-chlorite and illite evidently increased; in contrast, the content of kaolinite and mixed layer mineral decreased. There was obviously positive relationship between illite content and exchange K concentration in soil. Rice yield was distinctly minus correlative to illite content in fine clay, therein it illuminated that the availability of K in fine clay illite was higher than that in coarse clay illite. Change in soil minerals led to variance of ammonium adsorption with different treatments. According to the amount of ammonium adsorption, the rank with treatments was NPKOM > NPK > NP, CK > N, P, K > NK. SOC had positive action on the capacity of NH₄⁺ adsorption. However, with increasing NH₄⁺ ion concentration in soil solution (200-300 mg L^-1), the relative action of SOC could be diminished but the adsorption capacity of soil clay minerals was strengthened. The ammonium adsorption potential (M) was positively correlative to the poor crystal illite (PCI) content (P = 0.012). There was positive correlation between soil exchangeable K⁺ content and M(P = 0.007), except for NPKOM treatment. Soil organic carbon content and CEC did not have significant correlation with M.2) Effect of wetting and drying cycle on the mineralization of soil organic matter in the presence of microbe: Compared with continuous wetting treatment, drying period led to increase by 78%-204% of soil respiration rate, rewetting treatment after drying phase enhanced by 74%-95% of soil respiration rate and by 21% of N mineralization with NPK, whereas N mineralization decreased by 34%-78% in drying treatment. In rewetting treatment after drying phase N mineralization rate decreased by 5% and 13% with CK and NPKOM, respectively. Soil respiration rate was relative to microbial activity and growth, but mainly depended on soil organic content in the experiment. The slow drying of the paddy soils stimulated microbial growth and activity, whereas the rapidly rewetting of the dried soils caused a stress to microbial growth within a short time and induced microbial explosive growth in following days. Due to the microbial growth, inorganic N pool was reduced. Under the wetting condition, it was only in NPKOM treatment that soil pH value was markedly minus link to soil microbial biomass. The effect of drying treatment on soil pH value was not obviously different among treatments. Soil microbial biomass was increased with increasing soil pH value.3) Yield effect and used efficiency of fertilizers: Based on the mean rice yield, treatments with early rice could be separated into four levels, e.g. 2NPK, NPKOM > NPK > NP, NK > CK, N, P, K, PK; whereas that with late rice was 2NPK, NPKOM > NPK > NP, NK = N, P, PK, K, CK. Fertilization had cumulative effect on early rice yield, but it had no similar action on late rice. Only application of N fertilizer continuously decreased early rice yield over cropping time and the reducing variance are little by little big. The treatments without P or K also result in the cumulative trend of decreasing rice yield. Regardless of early rice or late rice, NPK treatment had the highest agronomic efficiency (AE) and recovery efficiency (RE) of fertilizers, and followed by NP and NK treatments; N treatment was the lowest one among treatments. Average agronomic efficiency of N or K was 1-12 kg grain kg^-1 N or K, respectively, and that of P was 10-30 kg grain kg^-1 P. RE-K (24%-71%) was higher than RE-P (19%-52%) and RE-N (5%-36%) for all treatments. As for early rice, AE-N with treatment applied N fertilizer decreased over cultivated time. P used efficiency with early rice increased over cropping time, but it was adverse for later rice. There was not significant K used efficiency with early rice or later rice.Main conclusions of the present research:Long-term no balance application of chemical fertilizers significantly changed soil clay minerals and other properties and influenced on rice yield and cumulative effect of fertilizers. K fertilization enhanced the content and crystal intensity of illite. The increment of illite concentration improved soil capability of ammonium adsorption. Fertilization free of organic manure led to the increase of soil anti-penetration resistance and consequently one of key factors to constrain rice yield. Transformation of field irrigation managements from long term wetting to wetting and drying cycle not only increased soil respiration rate in drying phase but also strengthened soil respiration rate in rewetting phase after drying. The present research illuminated that long-term application of chemical and organic fertilizers resulted in change in soil physical properties, and then influenced on transformation of soil nutrients and paddy soil productivity. However the function and mechanism of the process was needed to perform further research.