Three-source partitioning of soil respiration by ^(13)C natural abundance and its variation with soil depth in a plantation

Partitioning soil respiration into three components is vital to identify CO_2 sink or source and can help us better understand soil carbon dynamics. However, knowledge about the influences of soil depth and the priming effect on soil respiration components under field has been limited. Three components of soil respiration（root respiration, rhizomicrobial respiration and basal respiration） in a plantation in the hilly area of the North China were separated by the 13 C natural abundance method. The results showed that the average proportions of rhizomicrobial respiration, root respiration and basal respiration at the 25-65 cm depths were about 14, 23 and 63 %, respectively. Three components of soil respiration varied with soil depth, and root respiration was the main component of soil respiration in deeper soil. The priming effect was obvious for the deep soil respiration, especially at the 40-50 cm depth. Thus, depth and priming effect should be taken into account to increase the accuracy of estimations of soil carbon flux.

用自然^(13)C丰度法区分人工林根源呼吸的原理与应用

将土壤呼吸分为土壤微生物呼吸、纯根呼吸和根际微生物呼吸的研究对定量评价陆地生态系统地下碳平衡具有重要的意义。理论上,自然13C丰度法是区分林木根源呼吸组分的最佳方法,但目前却难以将其应用于野外实践。本研究以黄河小浪底刺槐人工林为研究对象,利用自然13C丰度法对该人工林纯根呼吸和根际微生物呼吸进行区分,分析不同月份纯根呼吸和根际微生物呼吸在土壤呼吸中所占比例的变化,讨论环境因子对区分纯根呼吸和根际微生物呼吸研究的影响。结果表明:7、8和9月植物旺盛生长季,纯根呼吸占根源呼吸的比例分别为43%、52%和27%;根际微生物呼吸占根源呼吸的比例分别为57%、48%和73%;土壤温度和电导率对纯根呼吸和根际微生物呼吸占土壤呼吸比例及其标准偏差(SD)有一定影响。

The hydrologic record of karst systems: linking soil moisture to the carbon isotope signatures of soils above the Blue Spring cave system

Speleothem carbonates are precipitated continuously from inorganic carbon dissolved in seepage water infiltrating from the land surface,that reflects a mixture of atmospheric CO_2,respired soil carbon,and epikarst sources,each with distinct δ^(13)C values.To aid in deconvolving these signatures,soil samples were collected above the Blue Spring cave system in Sparta,Tennessee,USA and subjected to a series of incubation experiments,in order to constrain the correlation between CO_2respiration rates and soil moisture.This relationship is used to parameterize a simple mixing model which predicts the relationship between δ^(13)C and soil moisture in fluids infiltrating into the underlying cave system.

Towards predicting biochar impacts on plant-available soil nitrogen content

Biochars can improve soil health but have been widely shown to reduce plant-available nitrogen(N)owing to their high carbon(C)content,which stimulates microbial N-immobilization.However,because biochars contain large amounts of C that are not microbially available,their total elemental C:N ratio does not correspond well with impacts on soil N.We hypothesized that impacts on soil plant-available N would relate to biochar mineralizable-C(C_(min))content,and that C:N ratios of the mineralizable biochar component could provide a means for predicting conditions of net soil N-mineralization or-immobilization.We conducted two laboratory experiments,the first measuring biochar C_(min)from respiration of isotopically labeled barley biochars manufactured at 300,500,and 750℃,and the second characterizing C_(min)by proxy measurements for ten biochars from six feedstocks at several temperatures.For both experiments,soils were incubated with 2%biochar by mass to determine impacts to soil N-mineralization.Contrary to expectation,all the biochars increased soil N-mineralization relative to unamended soils.Also unexpected,higher temperature(500 and 700℃)barley biochars with less C_(min)stimulated more soil decomposition and more soil N-mineralization than a 350℃barley biochar.However,across diverse biochar feedstocks and production methods,none of the biochar characteristics correlated with soil N-mineralization.The finding of improved soil N-mineralization adds complexity to the range of soil N responses that can be expected in response to biochar amendment.Because of the limited ability to predict soil N responses from biochar properties,users should monitor soil N to manage soil fertility.

Net ecosystem carbon exchange for Bermuda grass growing in mesocosms as affected by irrigation frequency

Intensification of grazed grasslands following conversion from dryland to irrigated farming has the potential to alter ecosystem carbon(C)cycling and affect components of carbon dioxide(CO_(2))exchange that could lead to either net accumulation or loss of soil C.While there are many studies on the effect of water availability on biomass production and soil C stocks,much less is known about the effect of the frequency of water inputs on the components of CO_(2)exchange.We grew Bermuda grass(Cynodon dactylon L.)in mesocosms under irrigation frequencies of every day(I_(1) treatment,30 d),every two days(I_(2) treatment,12 d),every three days(I_(3) treatment,30 d),and every six days(I_(6) treatment,18 d,after I_(2) treatment).Rates of CO_(2)exchange for estimating net ecosystem CO_(2)exchange(F_(N)),ecosystem respiration(R_(E)),and soil respiration(R_(S))were measured,and gross C uptake by plants(F_(G))and respiration from leaves(R_(L))were calculated during two periods,1–12 and 13–30 d,of the 30-d experiment.During the first 12 d,there were no significant differences in cumulative F_(N)(mean±standard deviation,61±30 g C m^(-2),n=4).During the subsequent 18 d,cumulative F_(N) decreased with decreasing irrigation frequency and increasing cumulative soil water deficit(W),with values of 70±22,60±16,and 18±12 g C m^(-2) for the I_(1),I_(3),and I_(6) treatments,respectively.There were similar decreases in F_(G),R_(E),and R_(L) with increasing W,but differences in R_(S) were not significant.Use of the C_(4) grass growing in a C_(3)-derived soil enabled partitioning of R_(S) into its autotrophic(R_(A))and heterotrophic(R_(H))components using a^(13)C natural abundance isotopic technique at the end of the experiment when differences in cumulative W between the treatments were the greatest.The values of R_(H) and its percentage contributions to R_(S)(43%±8%,42%±8%,and 8%±5%for the I_(1),I_(3),and I_(6) treatments,respectively)suggested that R_(H) remained unaffected across a wide range of W and then decreased under extreme W.There were no significant differences in aboveground biomass between the treatments.Nitrous oxide(N_(2)O)emission was measured to determine if there was a trade-off effect between irrigation frequency and increasing W on net greenhouse gas emission,but no significant differences were found between the treatments.These findings suggest that over short periods in well-drained soil,irrigation frequency could be managed to manipulate soil water deficit in order to reduce net belowground respiratory C losses,particularly those from the microbial decomposition of soil organic matter,with no significant effect on biomass production and N_(2)O emission.

The differences in carbon dynamics between boreal dwarf shrubs and Scots pine seedlings in a microcosm study

Aims The ground level of boreal pine forests consists of a dense layer of eri-caceous shrubs,herbs,grasses,mosses and lichens.The primary prod-uctivity of this forest floor vegetation is notable but the role the most common ericoid dwarf shrub plant species,Calluna vulgaris,Vaccinium myrtillus and Vaccinium vitis-idaea,play in carbon(C)cycling in these ecosystems is poorly understood.Here,we studied their C dynamics in detail using plants of similar size(age 14-19 months)in a microcosm study.Methods We determined the full C balances of these dwarf shrubs for the first time and compared them to those of Pinus sylvestris by using long-term biomass accumulation,^(13)C pulse labelling and CO_(2) ex-change measurements in a controlled laboratory experiment.Important Findings Pinus sylvestris had significantly higher biomass-based C fluxes than dwarf shrubs,both aboveground and belowground,but the dwarf shrubs did not differ in the biomass-based fluxes.We showed that root respiration of the evergreen ericoid dwarf shrubs was sensitive to the aboveground light conditions as belowground respiration was 50-70%higher under light compared with dark conditions.Such light-related dif-ferences were not observed for Scots pine.The observed differences in C dynamics are important in estimating the origin of belowground CO_(2) fluxes and in evaluating their biological relevance.Our results improve current understanding of CO_(2) sources and sinks in boreal ecosystems.