Effects Of Different Fertilization On Contents Of Soil Microbial Biomass And Soluble Carbon And Nitrogen On The Loess Plateau

Carbon and nitrogen cycles in soil were important processes in agroecological system, and affect the stability, productivity and surroundings of the ecosystem. Soil microbe plays an important role in these cycles. Microbial biomass in soil not only contains a labile pool of nutrients, but also drives the cycling of organic matter and nutrients in soil. Soluble organic matter (SOM) is another important and active component of organic matters in terrestrial and aquatic ecosystems. It plays a key role in the biogeochemistry of carbon and nitrogen, and in pedogenesis in soils. SOM is also one of the labile forms of N available to microorganisms and plants. Soluble organic carbon (SOC) and soluble organic nitrogen (SON) have important roles in regulating C and N cycles in terrestrial and aquatic ecosystems. A lot of scholars have studied the effect of kinds of culitivating factors on the contents of soil microbial biomass carbon and nitrogen (SMBC, SMBN), and their roles in adjusting nutrient supply to crops.The studys on SOC and SON were mainly concentrated on forest soil. Information available on the content changes of SMBC, SMBN, SOC and SON in soils under the continuous fallow, different rotation system is limited. And the relationships between soil microbial biomass and soil soluble matter remain obscure.Therefore, in the study, the contents and changes of SMBC, SMBN, SOC, SON and mineral nitrogen in the soils under different models of fertilization and cultivation in the farmland ecosystem were measured to evaluate the roles of SOC, SON, and SMBC, SMBN in cycles of soil nitrogen. The results showed that:(1) Straw mulching increased the content of soil microbial biomass nitrogen (SMBN) significantly; but plastic mulching decreased the content of SMBN. Both straw mulching and plastic mulching reduced the content of soil soluble organic nitrogen (SON) during wheat growth stage. Appropriate nitrogen application rate (120 kg N/hm2) was favorable to increasing the content of SMBN in the late stage of wheat growth; however, high application rate of N fertilizer (240 kg N/hm2) decreased the content of SMBN significantly. The average content of SON increased with application rate of mineral nitrogen in wheat growth period.(2) Compared with the treatment without addition of fertilizers (CK), the application of organic material with N, P, K fertilizers significantly increased the contents of SMBC, SMBN, SOC and SON in the 0-10, 10-20 cm soil layers during wheat growth stage, but different materials with different ratios of C and N had different effects on the immobilization and release of nitrogen through microbial biomass. The contents of SMBC and SMBN in the 0-10 soil layer of treatment of N, P, K fertilizers with straw (SNPK) were only 71.9-95.2% and 66.3-85.6% of N, P, K fertilizers with manure (MNPK) , respectively; but the content of mineral nitrogen in SNPK was higher than MNPK in all wheat growth stages. Only addition of chemical fertilizers (NPK) had minimal impact on the contents of SMBC and SMBN in soils, but significantly increased the contents of SOC and SON in soils during wheat growth stage, and decreased the percent of SON in total soluble nitrogen (TSN).(3) The continuous fallow treatment increased the contents of soil organic carbon and total nitrogen significantly. Compared with CK and NPK treatment, the contents of SMBC, SMBN, SOC and SON in the continuous fallow treatment were increased in the 0-10 cm soil layer, but not in the 10-20 cm soil layer.(4) Compared with the wheat and summer fallow rotation (WF), the wheat and maize rotation (WM) significantly increased the contents of SMBC, SMBN, SOC, SON, and the total organic carbon (TOC) and nitrogen (TN) in the soils of 0-10 cm layer, but its effect on the contents of these parameters in 10-20 cm was not significant. The continuous fallow also significantly increased the contents of SMBC, SMBN, SOC, SON, TOC and TN in 0-10 cm soils layer in comparison with the treatments without addition of fertilizer (CK) or addition of chemical fertilizer treatment (NPK) under both WF and WM crop rotation.(5) The contents of SMBC and SMBN was increased with the growth of wheat and maize, in the shooting stage of wheat and grain filling stage of maize, the contents of SMBC and SMBN reached the greatest; and then the contents of SMBC and SMBN decreased. But in the shooting stage of wheat and grain filling stage of maize, the contents of mineral nitrogen in MNPK, SNPK and NPK treatments were the lowest in soils.(6) Ratios of SMBC to total organic carbon was in range of 2.03%-4.10%, averagely 2.78% in the 0-20 cm soil layer in different fertilization treatments in the farmland; ratios of SMBN to total nitrogen was in range of 2.64%-5.25%, averagely 3.71%. The ratios of SMBC to TOC, SMBN to TN were higher in the 0-10 cm than 10-20 cm significantly. Crop rotation systems and fertilizer treatments had no significant effects of on the ratios of SMBC/TOC and SMBN/TN in soils. The contents of SMBC SMBN not only positively correlated with TOC and TN, but also positively correlated with SOC, SON.