Effect Of Long-term Fertilization On Soil Organic Carbon, Organic Nitrogen And Microbial Properties In Subtropical Paddy Soils

Soil organic matter,soil microbial biomass,soil microbial community functional diversity,and soil organic matter mineralization affected by the applications of inorganic fertilizers and organic amendments in subtropical paddy soils were investigated.Soil samples were collected from the plough layer of 8 long-term field experiments at Xinhua, Ningxiang,Taojiang,Zhuzhou,Hanshou,Linli,Nanxian,and Wugang counties in Hunan Province,China.The long-term field experiments began in 1986,with the sites receiving the treatments:(1) no fertilizer[CK](2) inorganic NPK fertilizer[NPK](3) medium rate manure along with NPK fertilizer[MOM](4) high rate manure along with NPK fertilizer [HOM](5) rice straw along with NPK fertilizer[STR](6) inorganic NK fertilizer[NK](7) inorganic NP fertilizer[NP].Results showed that:1.Compared with initial levels,soil total organic C(TOC) in CK treatment increased at all sites,and total N(TN) increased at three sites.Compared with CK,application of NPK alone showed no significant effects on soil TOC and TN.NPK plus manure or straw significantly increased soil TOC and TN content.Ratio of C to N of paddy soils were ranged from 9:1～17:1.The different treatments had no significant effects on these values,except the MOM and HOM treatments.In comparison with initial levels,long-term plantation altered the soil pH; Application of inorganic fertilizers or organic amendments caused a soil pH decrease, compared with CK.The observed pH changes were due to the acidification of inorganic fertilizer and additional organic amendments to soils.2.Compared with CK,application of manure or straw along with inorganic fertilizer significantly increased the contents of soluble organic C(SOC),light fraction organic C (LFOC),readily oxidation organic C(ROOC).The percentages of TOC presented as SOC, LFOC and ROOC were ranged from 0.13%～0.32%,3.8%～17.4%and 49.0%～58.3%, respectively.There was significant difference in these percentages for different treatments. There were highly significant correlations between SOC,LFOC,ROOC and TOC.3.Soil organic N was fractionated by acid hydrolysis-distillation method.Total acid hydrolysable N(TAHN) accounted for 64%～92%of the TN in the paddy soils. Compared with CK,application of inorganic fertilizer or organic amendments increased amino acid N(AAN),ammonium N(AN),and amino sugar N(ASN) and decreased hydrolysable unidentified nitrogen(HUN) for most of sites.Several treatments had no significant effects. Soil soluble organic N(SON) accounted for 0.4%～0.9%of TN.NPK plus manure or straw significant increased the contents and percentages of SON.SON was closely correlated with the soil organic N fractions.4.Soil microbial biomass(MB) C and N were measured by the fumigation-extraction method.Compared with CK,application of inorganic fertilizers alone showed no significant effects on soil MB C and MB N.Application of manure or straw along with inorganic fertilizers also significantly increased soil MB C and MB N contents for all sites.In average,soil MB C represented 2.1%of TOC and MB N accounted for 2.5%of TN.MBC and MBN were closely correlated with the soil TOC and TN contents respectively.The tendency for a reduction in MB C:N ratio in all the treatments compared with CK.The soil microbial community functional diversity was detected by BIOLOG system. The Average Well Color Development(AWCD) in BIOLOG plates indicated the ability of carbon utilization of microbial community.All the treatments exhibited the elevation of AWCD during the period of incubation,but the differences among the treatments were obvious.Application of manure along with inorganic fertilizer caused the highest increase of the AWCD while applying straw-incorporation had less affection on the AWCD, which was even lower than the CK treatment.It implicated that long-term fertilization resuked in the variation of the carbon utilization efficiency of soil microbial communities.The indices of Shannon,Simpson and McIntosh were calculated to show the richness, dominance and evenness of the functional diversity,while the principal component analysis of substrate reactions reflected the main carbon sources utilized by microbial community.The treatments of manure along with inorganic fertilizer remarkably improved the richness,dominance and evenness of soil microbial community in comparison with CK.The treatment of straw-incorporation induced increase of the dominance,but had no significant effects on the richness and evenness.The principal components were extracted from the principal component analysis and their cumulative contribution of variance accounted to 85%～92%.The correlation coefficients between main substrates and PC1 or PC2 indicated that the main carbon sources for soil microbes were carbohydrates and amino acids in all treatments and there were significant differences of carbon substrate utilization patterns in different treatments.5.Dynamics of soil organic C and N mineralization affected by different treatments and its relationship with organic C fractions in paddy soils were investigated.Mineralization of soil organic C was estimated by using 58-d anaerobic incubation at 30℃.The cumulative amounts of mineralized CO₂ ranged from 448.64～1516.77μg/g and mineralized CH₄ ranged from 15.60～33.34μg/g.Total amounts accounted for 3.59～5.57%of soil TOC.The rates of CO₂ production were higher during the 13-day incubation for different treatments,then decreased and afterwards a little varied.The rates of CH₄ production presented a slowly increasing at first and then rapidly declined to low constant rates.The cumulative released C(CRC) in NPK plus manure or straw treatments were greater than that in NPK treatments.Significant correlations were found between CRC and different organic C fractions.However,the percentages of ROOC,HC,and FC in TOC were positively correlated with the CRC.The correlations of soil organic C fractions with the percentages of CRC in TOC were not significant.Mineralization of soil organic C was estimated by using 33-d aerobic incubation under 3 temperature regimes(10℃,20℃,30℃).The rates of CO₂ production were higher during the earlier phase(0～13 d) for different treatments,then decreased and afterwards a little varied.Daily mineralization of soil organic C during incubation accords to relationship of logarithm.While the soil temperature increasing,the amounts of total mineralized C increased for each treatment of each site.The amounts of total mineralized C in NPK plus manure or straw treatments were greater than that in inorganic fertilizers treatments.The correlations of different soil C fractions with total mineralized C were significantly correlated at 10℃,20℃and 30℃.The Q₁₀ values of paddy soils for different treatments ranged from 1.01～1.53.Significant correlations were found between qCO₂,enzyme activities and Q₁₀,while there were negative correlations between C/N ratios,Kos and Q₁₀.Mineralization of soil organic C was estimated by using 21-d incubation under 4 temperature regimes(10℃,20℃,30℃,45℃) and 2 water contents(submerged and 65% water-filled pore spaces).Mineralization enhanced gradually with temperature in the range of 10-30℃.Compared with submerged incubation,the condition of 65% water-filled pore spaces was more beneficial to soil N mineralization.Mineralization of soil organic N was estimated by using 98-d anaerobic incubation at 30℃.The amounts of net mineralized N(NMN) increased with the prolongation of incubation time.The rates of N mineralization were highest about at the 4th week incubation for different treatments.The rate of mineralization and the ratio of NMN to TN in NPK plus manure or straw treatments are greater than that of inorganic fertilizers treatments.Significant correlations were found between NMN and TN and MBN.The data obtained show that the soil organic matter and microbial activities in paddy soils was improved by application of manure along with inorganic fertilizers when compared to inorganic fertilizers alone.It is therefore important that the traditional use of manure should be strongly promoted in order to maintain long-term soil productivity even where inorganic fertilizers are being used to achieve high yields.Not only will this improve the soil organic matter but it will also reduce the rates of inorganic fertilizers required since nutrients will be released from the manures.The incorporation of rice straw is another important way of promoting soil organic matter accumulation,so the importance of straw retention also needs to be stressed.