Topsoil organic carbon mineralization and CO_2 evolution of three paddy soils from South China and the temperature dependence

Carbon mineralization and its response to climatic warming have been receiving global attention for the last decade. Although the virtual influence of temperature effect is still in great debate, little is known on the mineralization of organic carbon （SOC） of paddy soils of China under warming. SOC mineralization of three major types of China＇s paddy soils is studied through laboratory incubation for 114 d under soil moisture regime of 70% water holding capacity at 20℃ and 25℃ respectively. The carbon that mineralized as CO2 evolved was measured every day in the first 32 d and every two days in the following days. Carbon mineralized during the 114 d incubation ranged from 3.51 to 9.22 mg CO2-C/gC at 20℃ and from 4.24 to 11.35 mg CO2-C/gC at 25℃ respectively; and a mineralizable C pool in the range of 0.24 to 0.59 gC/kg, varying with different soils. The whole course of C mineralization in the 114 d incubation could be divided into three stages of varying rates, representing the three subpools of the total mineralizable C： very actively mineralized C at 1-23 d, actively tnineralized C at 24--74 d and a slowly mineralized pool with low and more or less stabilized C mineralization rate at 75-114 d. The calculated Q10 values ranged from 1.0 to 2.4, varying with the soil types and N status. Neither the total SOC pool nor the labile C pool could account for the total mineralization potential of the soils studied, despite a well correlation of labile C with the shortly and actively mineralized C, which were shown in sensitive response to soil warming. However, the portion of microbial C pool and the soil C/N ratio controlled the C mineralization and the temperature dependence. Therefore, C sequestration may not result in an increase of C mineralization proportionally. The relative control of C bioavailability and microbial metabolic activity on C mineralization with respect to stabilization of sequestered C in the paddy soils of China is to be further studied.

Linking changes in the soil microbial community to C and N dynamics during crop residue decomposition

Crop residues are among the main inputs that allow the organic carbon(C)and nutrients to be maintained in agricultural soil.It is an important management strategy that can improve soil fertility and enhance agricultural productivity.This work aims to evaluate the extent of the changes that may occur in the soil heterotrophic microbial communities involved in organic matter decomposition and C and nitrogen(N)mineralization after the addition of crop residues.Soil microcosm experiments were performed at 28℃ for 90 days with the addition of three crop residues with contrasting biochemical qualities:pea(Pisum sativum L.),rapeseed(Brassica napus L.),and wheat(Triticum aestivum L.).Enzyme activities,C and N mineralization,and bacterial and fungal biomasses were monitored,along with the bacterial and fungal community composition,by the highthroughput sequencing of 16 S rRNA and ITS genes.The addition of crop residues caused decreases in β-glucosidase and arylamidase activities and simultaneous enhancement of the C mineralization and net N immobilization,which were linked to changes in the soil microbial communities.The addition of crop residues decreased the bacterial and fungal biomasses 90 days after treatment and there were shifts in bacterial and fungal diversity at the phyla,order,and genera levels.Some specific orders and genera were dependent on crop residue type.For example,Chloroflexales,Inquilinus,Rubricoccus,Clitocybe,and Verticillium were identified in soils with pea residues;whereas Thermoanaerobacterales,Thermacetogenum,and Hypoxylon were enriched in soils with rapeseed residues,and Halanaerobiales,Rubrobacter,and Volutella were only present in soils with wheat residues.The findings of this study suggest that soil C and N dynamics in the presence of the crop residues were driven by the selection of specific bacterial and fungal decomposers linked to the biochemical qualities of the crop residues.If crop residue decomposition processes showed specific bacterial and fungal operational taxonomic unit(OTU)signatures,this study also suggests a strong functional redundancy that exists among soil microbial communities.

Soil under dead or live organic matter systems:Effect of European shag(Phalacrocorax aristotelis L.)nesting on soil nematodes and nutrient mineralization

Here we studied whether soil systems differ if they are under the influence of live(plants)or dead organic matter systems(nest)in terms of C and N mineralization,microbiological characteristics and nematode trophic group structure.We analyzed physicochemical and microbiological properties of soils inside and outside nests of the European shag(Phalacrocorax aristotelis L.)on the Cies Islands(NW Spain).We sampled fresh soil below dead(nests)and live organic matter(plants)(paired samples,n=7).Soil of nests had lower organic matter and higher electric conductivity and dissolved organic C and extractable N contents than the soil of plants.Microbial biomass and activity were greater in soil of nests than in soil of plants.The abundance of nematode trophic groups(bacterivores,fungivores,omnivores and herbivores)differred between soils of nests and plants,and the soil of plants supported a more abundant and diverse nematode community.The present results points to that source of organic matter promote differences in the decomposer community,being more efficient in soil of nests because C mineralization is greater.Further,this occurred independently of the complexity of the systems,higher in the soil of plants with more groups of nematodes.