Key Physical Factors Affecting Spatial-temporal Variation of Labile Organic Carbon Fractions by Biochar Driven in Mollisols Region of Northeast China

Biochar is widely used to improve soil physical properties and carbon sequestration. However, few studies focuse on the impact of maize stalk biochar on labile organic carbon(LOC) pool and the relationship between physical properties and LOC fractions. A field positioning experiment was performed in Mollisols region of Northeast China to evaluate the influence of maize stalk biochar on the spatial distribution and temporal changes of physical properties and LOC fractions. Maize stalk biochar treatments included C1(1.5 kg·hm^(-2)), C2(3 kg·hm^(-2)), C3(15 kg·hm^(-2)), C4(30 kg·hm^(-2)), and CK(0). The results showed that maize stalk biochar increased soil water contents(SWC) and soil porosity(SP), but reduced bulk density(BD). Maize stalk biochar reduced dissolved organic carbon(DOC) contents in the 0-20 cm soil layer, ranging from 0.25 g·kg^(-1) to 0.31 g·kg^(-1) in harvest period, while increased in the 20-40 cm soil layer. In addition, the application of biochar had a significant impact on the spatial distribution and temporal change of SWC, BD, SP, DOC, hot-water extractable carbon(HWC), acid hydrolyzed organic carbon(AHC Ⅰ, Ⅱ), and readily oxidized organic carbon(ROC). High amounts of maize stalk biochar up-regulated the contents of soil organic carbon SOC, HWC, AHC Ⅰ, AHC Ⅱ, and ROC. In addition, SWC and SP were the key physical factors to affect LOC fractions. In conclusions, maize stalk biochar could improve physical properties, and then influence LOC fractions, and maize stalk biochar could be used as an organic amendment for restoring degraded soils governed by their rates of addition.

Can soil organic carbon sequestration and the carbon management index be improved by changing the film mulching methods in the semiarid region?

Plastic film mulching has been widely used to increase maize yield in the semiarid area of China.However, whether long-term plastic film mulching is conducive to agricultural sustainability in this region remains controversial.A field experiment was initiated in 2013 with five different film mulching methods:(i) control method, flat planting without mulching (CK),(ii) flat planting with half film mulching (P),(iii) film mulching on ridges and planting in narrow furrows(S),(iv) full film mulching on double ridges (D), and (v) film mulching on ridges and planting in wide furrows (R).The effects on soil organic carbon (SOC) content, storage, and fractions, and on the carbon management index (CMI)were evaluated after nine consecutive years of plastic film mulching.The results showed that long-term plastic film mulching generally maintained the initial SOC level.Compared with no mulching, plastic film mulching increased the average crop yield, biomass yield, and root biomass by 48.38, 35.06, and 37.32%, respectively, which led to the improvement of SOC sequestration.Specifically, plastic film mulching significantly improved CMI, and increased the SOC content by 13.59%, SOC storage by 7.47%and easily oxidizable organic carbon (EOC) by 13.78%on average,but it reduced the other labile fractions.SOC sequestration and CMI were improved by refining the plastic film mulching methods.The S treatment had the best effect among the four mulching methods, so it can be used as a reasonable film mulching method for sustainable agricultural development in the semiarid area.

Effects of stand density on soil respiration and labile organic carbon in different aged Larix principis-rupprechtii plantations

Background:The carbon pools of forest soils play a vital role in global carbon sequestration and emissions.Forest management can regulate the sequestration and output of forest soil carbon pools to a certain extent;however,the kinetics of the effects of forest density on soil carbon pools require further investigation.Methods:We established sample plots with stand density gradients in three different aged Larix principis-rupprechtii plantations and quantified the soil respiration,soil organic carbon(SOC),soil dissolved organic carbon(DOC),microbial biomass carbon(MBC),light fraction organic carbon(LFOC),and readily oxidized carbon(ROC).Results and conclusions:During the growth and development of plantations,stand density is an essential factor that impacts soil respiration and its associated elements.Moderate density was observed to promote both the soil and heterotrophic respiration rates and the sequestration of MBC and LFOC,whereas it inhibited the sequestration of ROC.The soil,heterotrophic,and autotrophic respiration rates of older forest stands were relatively rapid,whereas the contents of SOC,MBC,LFOC,DOC,and ROC were higher and more sensitive to changes in stand density.The MBC,LFOC,and ROC in soil labile organic carbon were closely related to both the soil and heterotrophic respiration,but not the SOC.Among them,the LFOC and MBC played the roles of“warehouse”and“tool”and were significantly correlated with soil and heterotrophic respiration.The ROC,as a“raw material”,exhibited a significantly negative correlation with the soil and heterotrophic respiration.When the soil and heterotrophic respiration rates were rapid,the ROC content in the soil maintained the low level of a“dynamically stabilized”state.The stand density regulated heterotrophic respiration by affecting the soil labile organic carbon,which provided an essential path for the stand density to regulate soil respiration.

Corn straw return can increase labile soil organic carbon fractions and improve water-stable aggregates in Haplic Cambisol

Corn straw return to the field is a vital agronomic practice for increasing soil organic carbon(SOC)and its labile fractions,as well as soil aggregates and organic carbon(OC)associated with water-stable aggregates(WSA).Moreover,the labile SOC fractions play an important role in OC turnover and sequestration.The aims of this study were to determine how different corn straw returning modes affect the contents of labile SOC fractions and OC associated with WSA.Corn straw was returned in the following depths:(1)on undisturbed soil surface(NTS),(2)in the 0–10 cm soil depth(MTS),(3)in the 0–20 cm soil depth(CTS),and(4)no corn straw applied(CK).After five years(2014–2018),soil was sampled in the 0–20 and 20–40 cm depths to measure the water-extractable organic C(WEOC),permanganate oxidizable C(KMnO4-C),light fraction organic C(LFOC),and WSA fractions.The results showed that compared with CK,corn straw amended soils(NTS,MTS and CTS)increased SOC content by 11.55%–16.58%,WEOC by 41.38%–51.42%,KMnO4-C and LFOC by 29.84%–34.09%and 56.68%–65.36%in the 0–40 cm soil depth.The LFOC and KMnO4-C were proved to be the most sensitive fractions to different corn straw returning modes.Compared with CK,soils amended with corn straw increased mean weight diameter by 24.24%–40.48%in the 0–20 cm soil depth.The NTS and MTS preserved more than 60.00%of OC in macro-aggregates compared with CK.No significant difference was found in corn yield across all corn straw returning modes throughout the study period,indicating that adoption of NTS and MTS would increase SOC content and improve soil structure,and would not decline crop production.

Effect of fire intensity on active organic and total soil carbon in a Larix gmelinii forest in the Daxing'anling Mountains,Northeastern China

Studying contents and seasonal dynamics of active organic carbon in the soil is an important method for revealing the turnover and regulation mechanism of soil carbon pool. Through 3 years of field sampling and lab analysis, we studied the seasonal variations, content differences, and interrelationships of total organic carbon(TOC), light fraction organic carbon(LFOC), and particulate organic carbon(POC) of the soil in the forest areas burned with different fire intensities in the Daxing'anling Mountains. The mean TOC content in the low-intensity burned area was greater than that in the unburned area,moderate-intensity, and high-intensity burned areas in June and November(P<0.05). LFOC and POC in the lowintensity burned area were greater than that in either moderate-intensity or high-intensity burned areas, with significant differences in LFOC in September and November(P <0.05). A significant difference in LFOC between the unburned and burned areas was only found in July(P < 0.05). However, the differences in POC between the unburned and burned areas were not significant in all the whole seasons(P >0.05). Soil LFOC and POC varied significantly with the seasons(P< 0.05) in the Daxing'anling Mountains. Significant linear relationships were observed between soil TOC, LFOC, and POC, which were positively correlated with soil nitrogen and negatively correlated with soil temperature in the Daxing'anling Mountains.

Organic mulching promotes soil organic carbon accumulation to deep soil layer in an urban plantation forest

Background:Soil organic carbon(SOC)is important for soil quality and fertility in forest ecosystems.Labile SOC fractions are sensitive to environmental changes,which reflect the impact of short-term internal and external management measures on the soil carbon pool.Organic mulching(OM)alters the soil environment and promotes plant growth.However,little is known about the responses of SOC fractions in rhizosphere or bulk soil to OM in urban forests and its correlation with carbon composition in plants.Methods:A one-year field experiment with four treatments(OM at 0,5,10,and 20 cm thicknesses)was conducted in a 15-year-old Ligustrum lucidum plantation.Changes in the SOC fractions in the rhizosphere and bulk soil;the carbon content in the plant fine roots,leaves,and organic mulch;and several soil physicochemical properties were measured.The relationships between SOC fractions and the measured variables were analysed.Results:The OM treatments had no significant effect on the SOC fractions,except for the dissolved organic carbon(DOC).OM promoted the movement of SOC to deeper soil because of the increased carbon content in fine roots of subsoil.There were significant correlations between DOC and microbial biomass carbon and SOC and easily oxidised organic carbon.The OM had a greater effect on organic carbon fractions in the bulk soil than in the rhizosphere.The thinnest(5 cm)mulching layers showed the most rapid carbon decomposition over time.The time after OM had the greatest effect on the SOC fractions,followed by soil layer.Conclusions:The frequent addition of small amounts of organic mulch increased SOC accumulation in the present study.OM is a potential management model to enhance soil organic matter storage for maintaining urban forest productivity.

Straw addition increases enzyme activities and microbial carbon metabolism activities in bauxite residue

Recovery of microbial functions is one of the critical processes in the nutrient cycling of bauxite residue for improving revegetation.Straw is considered to be effective to increase microbial diversity and drive the development of the microbial community,but its effect on microbial carbon metabolism has not been illustrated.The present study evaluated the effects of phosphogypsum(PG),straw(SF)and phosphogypsum plus straw(PGSF)on physicochemical properties,enzyme activities,and microbial carbon metabolism activities in bauxite residue.After 180 days incubation,PG,SF and PGSF treatment significantly reduced the residue pH from 10.85 to 8.64,9.39 and 8.06,respectively.Compared to CK treatment,SF treatment significantly increased the content of total organic carbon(TOC)and organic carbon fractions(DOC,MBC,EOC,and POC).In addition,straw addition significantly increased glucosidase,cellulose,urease,and alkaline phosphatase by 7.2-9.1 times,5.8-7.1 times,11.1-12.5 times,and 1.1-2.2 times,respectively.The Biolog results showed that straw addition significantly increased microbial metabolic activity(AWCD)and diversity in bauxite residue.Redundancy analysis indicated total nitrogen(TN)and carbon fractions(POC,MBC and DOC)were the most important environmental factors affecting microbial metabolic activity and diversity in bauxite residue.These findings provided us with a biogeochemical perspective to reveal soil formation in bauxite residue and suggested that nutrient supplement and regulation of salinity-alkalinity benefit the establishment of microbial communities and functions in bauxite residue.

Variations of soil organic carbon fractions in response to conservative vegetation successions on the Loess Plateau of China

Land use changes profoundly affect the equilibrium of soil organic carbon(soC)sequestration and greenhouse gas emissions.With the current global climatic changes,it is vital to understand the influence of ecological restoration and conservation management on the dynamics of soC under different land uses,especially in erosion-endangered Loess soils.Therefore,we investigated changes in soc through a suit of labile fractions,namely:light fraction organic C(LFOC),heavy fraction organic C(HFOC),coarse particulate organic C(CPOC),fine particulate organic C(FPOC),and dissolved organic C(DOC),from two forests i.e.,Robinia pseudoacacia(RP)and Platycladus orientalis(PO),with different ages,in comparison with farmland(FL).The SOC and STN contents significantly increased over 42 years in the RP forest where the contents of CPOC and FPOC were significantly higher than in the FL.Moreover,total SOC and its labile fractions,in the studied land use types,significantly correlated with soil CacO3,pH,and STN contents,indicating their key roles in SoC sequestration.The results reported here from different vegetation with different ages provide a better understanding of sOC and STN alterations at different stages of vegetation restoration.Our findings suggest that long-term natural vegetation restoration could be an effective approach for SoC sequestration and soil conservation on the Loess soil.

Distribution of organic carbon fractions in soil aggregates in Chinese fir plantations with different stand ages

Background:Revealing the variations in soil aggregate-related organic carbon(OC)and labile organic carbon(LOC)fractions in a chronosequence of Chinese fir plantations plays an important role in better understanding the impact of soil carbon sink or source on the Chinese fir plantation ecosystem.In this study,soil samples in a depth of 0–20 cm were collected from Chinese fir plantations at different stand ages(0,9,17,and 26 years old)in Guangxi,China.With the optimal moisture sieving method adopted,the soil aggregates of 4 different sizes were obtained,including>2-mm,2–1-mm,1–0.25-mm,and<0.25-mm aggregates.Soil OC and LOC fractions were measured in the aggregates of different sizes.The LOC fractions included readily oxidizable carbon(ROC),particulate organic carbon(POC),microbial biomass carbon(MBC),water-soluble organic carbon(WOC),and mineralized organic carbon(MOC).Results:Soil aggregate stability,as indicated by the mean weight diameter(MWD),was the highest in the 17-yearold Chinese fir plantations and was significantly positively related(p<0.05)to the concentrations of OC and LOC fractions(except for the ROC and MOC),with the POC in particular.As for all stand ages of Chinese fir plantations,the concentrations of soil OC and LOC fractions were significantly increased as the aggregate size decreased.Consequently,there were more OC and LOC fractions distributed in the<0.25-mm aggregates.During the stand development,the concentrations of soil OC and LOC fractions first increased and then decreased,with the highest levels detected in the 17-year-old Chinese fir plantations,indicating that the 17-year-old Chinese fir plantations were conducive to the accumulation of soil OC and LOC fractions.Conclusion:After 17 years of planting,promoted soil carbon(especially for the POC)accumulation contributes significantly to enhancing soil aggregate stability for the Chinese fir plantations in Guangxi,China.

Effects of precipitation on soil organic carbon fractions in three subtropical forests in southern China

Aims The aim of this study was to investigate the effects of precipitation changes on soil organic carbon(SOC)fractions in subtropical forests where the precipitation pattern has been altered for decades.Methods We conducted field manipulations of precipitation,including ambient precipitation as a control(CK),double precipitation(DP)and no precipitation(NP),for 3 years in three forests with different stand ages(broadleaf forest[BF],mixed forest[MF]and pine forest[PF])in subtropical China.At the end of the experiment,soil samples were collected to assay SOC content,readily oxidizable organic carbon(ROC)and non-readily oxidizable organic carbon(NROC),as well as soil microbial biomass carbon(MBC),pH and total nitrogen content.Samples from the forest floors were also collected to analyze carbon(C)functional groups(i.e.alkyl C,aromatic C,O-alkyl C and carbonyl C).Furthermore,fine root biomass was measured periodically throughout the experiment.Important Findings Among the forests,ROC content did not exhibit any notable differences,while NROC content increased significantly with the stand age.This finding implied that the SOC accumulation observed in these forests resulted from the accumulation of NROC in the soil,a mechanism for SOC accumulation in the mature forests of southern China.Moreover,NP treatment led to significant reductions in both ROC and NROC content and therefore reduced the total SOC content in all of the studied forests.Such decreases may be due to the lower plant-derived C inputs(C quantity)and to the changes in SOC components(C quality)indicated by C functional groups analyses under NP treatment.DP treatment in all the forests also tended to decrease the SOC content,although the decreases were not statistically significant with the exception of SOC and ROC content in PF.This finding indicated that soils in MF and in BF may be more resistant to precipitation increases,possibly due to less water limitations under natural conditions in the two forests.Our results therefore highlight the different responses of SOC and its fractions to precipitation changes among the forests and suggest that further studies are needed to improve our understanding of SOC dynamics in such an important C sink region.