Soil Organic Carbon Turnover Rate Research Articles

Modelling soil organic carbon in Danish agricultural soils suggests low potential for future carbon sequestration

Soil organic carbon (SOC) is in active exchange with the atmosphere. The amount of organic carbon (OC) input into the soil and SOC turnover rate are important for predicting the carbon (C) sequestration potential of soils subject to changes in land-use and climate. The C-TOOL model was developed to simulate the dynamics of SOC storage on medium- to long-term trends in the whole soil profile (0–100cm), and was used to compare SOC changes under typical Danish farming conditions for two sites in Denmark having the greatest possible temperature differences for the period 1986 and 2012. For this purpose, various agricultural management scenarios were considered including characteristic crop rotations with and without the presence of cover crops, and the application of organic amendments. We compared these simulated management effects with management effects estimated from Danish SOC monitoring network over the same period of time. The results of the C-TOOL simulations demonstrated that application of organic manure, use of cover crop, and converting the croplands to grassland had the potential to increase SOC in Danish mineral soils. The simulated data also suggested that C-TOOL gave a reasonably good account of SOC changes when compared with measured SOC values from a Danish national monitoring network, and the simulated management effects agree well with results of long-term experimental trials. The estimates show rather small potential for increasing SOC in Danish cropping systems, even when there is conversion to rotations with more grasslands or the use of cover crops is increased.

Crop species affect soil organic carbon turnover in soil profile and among aggregate sizes in a Mollisol as estimated from natural 13C abundance

Background and aims Soil organic carbon (SOC) stock varies in soil profiles under different cropping systems, which is possibly due to the difference in SOC turnover rate. This study used a 22-year field experiment to determine the effect of crop species on SOC turnover rate in soil aggregates and profiles of a Mollisol. Methods Soils were sampled from 2-m soil profiles in 1991 and 2012, from the plough layer regularly under continuous monocropping of maize, soybean and wheat. Natural 13 C abundance was determined to calculate the fraction of newly derived C in SOC in bulk soil and water-stable aggregates of different sizes. Results The SOC content in the plough layer tended to decrease under wheat and maize but did not change under soybean from 1991 to 2012. The soil δ 13 C abundance in bulk soil and all aggregate sizes increased in 0–1.8 m depth under maize but decreased in 0–0.8 m under soybean and wheat, with the variations being larger in the larger aggregates. The SOC turnover at 0– 0.8 m depth was fastest under wheat, followed by soybean and slowest under maize. The SOC turnover was faster in the larger aggregates for all crops, with the variations among the crop species being larger in> 2 mm aggregates. Conclusions Crop species affect the δ 13 C abundance and SOC turnover rate consistently in different aggregate sizes in the plough layer and in bulk soil within the rooting zone.

Soil organic carbon stocks and CO2 effluxes of native and exotic pine plantations in subtropical China

The selection of tree species is critical for soil organic carbon (SOC) stock and dynamics under afforestation. Exotic tree species were widely introduced and have been considered as a form of human-induced global change. However, how exotic tree species, as compared with native ones, affect SOC storage and soil respiration (Rs) has not been sufficiently investigated. Here we studied this issue in an even-aged (22years) plantation composed by native Masson pine (Pinus massoniana Lamb.) (MP) and exotic slash pine (Pinus elliottii Engelm.) (SP) in subtropical China. A trenching method was used to partition Rs into heterotrophic (Rh) and autotrophic respiration (Ra). Rs and Rh were measured approximately biweekly during August 2007 to December 2009. Our results showed that seasonal patterns of Rs and Rh in both pine forests were mainly affected by soil temperature and moisture (R2=0.56–0.78, P<0.001). There was no difference in SOC stock (forest floor+mineral soil of 0–40cm), Rh and turnover rate (Rh/SOC stock) between MP and SP forests. However, annual Rs, Ra and Ra/Rs of MP were significantly 24%, 119% and 74% higher than those of SP, respectively. In contrast, even though MP and SP had similar fine and coarse root biomass, MP had significant higher ratios of Ra/fine root biomass and Ra/coarse root biomass than SP, suggesting a higher respiration rate in native pine species on a root biomass basis. We concluded that the exotic SP, as compared with indigenous MP, did not alter SOC stock, Rh and turnover rate but depressed Ra and Rs. Therefore, both pine species were recommended in case of reforestation in terms of SOC sequestration.

Warming-related increases in soil CO2 efflux are explained by increased below-ground carbon flux

The universally observed exponential increase in soil-surface CO2 effux (‘soil respiration’; FS) with increasing temperature has led to speculation that global warming will accelerate soil organic carbon (SOC) decomposition, reduce SOC storage, and drive a positive feedback to future warming. However, interpreting temperature–FS relationships, and so modelling terrestrial carbon balance in a warmer world, is complicated by the many sources of respired carbon that contribute to FS and a poor understanding of how temperature influences SOC decomposition rates. Here we quantified FS, litterfall, bulk SOC and SOC fraction size and turnover, and total below-ground carbon flux (TBCF) across a highly constrained 5.2 degree C mean annual temperature (MAT) gradient in tropical montane wet forest. From these, we determined that: increases in TBCF and litterfall explain >90% of the increase in FS with MAT; bulk SOC and SOC fraction size and turnover rate do not vary with MAT; and increases in TBCF and litterfall do not influence SOC storage or turnover on century to millennial timescales. This gradient study shows that for tropical montane wet forest, long-term and whole-ecosystem warming accelerates below-ground carbon processes with no apparent impact on SOC storage.

Aggregate stability and aggregate-associated carbon in no-till and reduced tillage practice in Finland

No-till (NT) and reduced tillage (RT) treatments can potentially enhance soil organic carbon (SOC) stabilization and soil aggregation. The turnover rate of SOC is affected by soil aggregate stability and the distribution of SOC across different aggregate-associated soil fractions. Carbon turnover rate slows down when soil aggregation increases and C is protected within stable microaggregates (53–250 μm). We studied the long-term (~10 yrs) effect of NT and RT practice on SOC stabilization in four soils typical for the boreal region. Distribution of SOC in different soil fractions was analyzed by wet sieving and further isolating microaggregates (mM) from large (&gt;2000 μm, LM) and small (250–2000 μm, sM) macroaggregates. Aggregate stability decreased in the order of NT &gt; RT &gt; CT at all study sites. A general trend of redistribution of SOC was found at all study sites, i.e., the LM fraction gained SOC whereas the sM fraction lost SOC under NT compared to CT, while three out of the four sites did not show any significant changes in total SOC content under NT compared to CT. Our results show no increase in the amount of mM fraction in NT practice. On the other hand, our hypothesis that there would be more SOC incorporated in mM fraction in NT and RT treatments compared to CT practice was corroborated at three of the four study sites. Overall, our results indicate that the potential to accumulate SOC under NT or RT compared to CT is limited in boreal agroecosystems.

Aggregate stability and aggregate-associated carbon in no-till and reduced tillage practice in Finland

No-till (NT) and reduced tillage (RT) treatments can potentially enhance soil organic carbon (SOC) stabilization and soil aggregation. The turnover rate of SOC is affected by soil aggregate stability and the distribution of SOC across different aggregate-associated soil fractions. Carbon turnover rate slows down when soil aggregation increases and C is protected within stable microaggregates (53–250 μm). We studied the long-term (~10 yrs) effect of NT and RT practice on SOC stabilization in four soils typical for the boreal region. Distribution of SOC in different soil fractions was analyzed by wet sieving and further isolating microaggregates (mM) from large (>2000 μm, LM) and small (250–2000 μm, sM) macroaggregates. Aggregate stability decreased in the order of NT > RT > CT at all study sites. A general trend of redistribution of SOC was found at all study sites, i.e., the LM fraction gained SOC whereas the sM fraction lost SOC under NT compared to CT, while three out of the four sites did not show any significant changes in total SOC content under NT compared to CT. Our results show no increase in the amount of mM fraction in NT practice. On the other hand, our hypothesis that there would be more SOC incorporated in mM fraction in NT and RT treatments compared to CT practice was corroborated at three of the four study sites. Overall, our results indicate that the potential to accumulate SOC under NT or RT compared to CT is limited in boreal agroecosystems.

Belowground carbon pools and dynamics in China's warm temperate and sub-tropical deciduous forests

Abstract. We report the first estimates of pools and dynamics of microbes, roots, plant litter and soil organic carbon (SOC) in three dominant types of China's vast deciduous forest area: Betula platyphylla, Quercus liaotungensis, and Quercus aliena var. acuteserrata. Organic matter degradation rates overshadowed litter inputs as the main determinant of the soil carbon stocks. Across the three forests, rates of litter decomposition were also indicative for turnover rates of SOC. Litter and SOC decay was faster in the sub-tropical than in the warm-temperate forests. Among the latter, SOC turnover was highest in the forest producing the higher-quality litter. Microbial biomass was, as expected, correlated with SOC content. Microbial activity, in contrast, was highest at the sub-tropical forest, despite the lower SOC availability, lower fraction of labile SOC, and lower soil microbial biomass. These results may contribute to increased understanding of controls over belowground carbon cycling in deciduous forests.

Turnover Rate of Soil Organic Matter and Origin of Soil 14Co2 in Deep Soil from a Subtropical Forest in Dinghushan Biosphere Reserve, South China

This paper examines the carbon isotopes (13C, 14C) of soil organic carbon (SOC) and soil CO2 from an evergreen broadleaf forest in southern China during the rainy season. The distribution of SOC δ13C, and SOC content with depth, exhibits a regular decomposition of SOC compartments with different turnover rates. Labile carbon is the main component in the topsoil (0–12 cm) and has a turnover rate between 0.1 and 0.01 yr–1. In the middle section (12–35 cm), SOC was mainly comprised of mediate carbon with turnover rates ranging between 0.01 and 0.025. Below 35 cm depth (underlayer section), the SOC turnover rate is slower than 0.001 yr–1, indicating that passive carbon is the main component of SOC in this section. The total production of humus-derived CO2 is 123.84 g C m–2 yr–1, from which 88% originated in the topsoil. The middle and underlayer sections contribute only 10% and 2% to the total humus-derived CO2 production, respectively. Soil CO2 δ13C varies from –24.7‰ to –24.0‰, showing a slight isotopic depth gradient. Similar to soil CO2 δ13C, Δ14C values, which range from 100.0‰ to 107.2‰, are obviously higher than that of atmospheric CO2 (60–70‰) and SOC in the middle and underlayer section, suggesting that soil CO2 in the profile most likely originates mainly from SOC decomposition in the topsoil. A model of soil CO2 Δ14C indicates that the humus-derived CO2 from the topsoil contributes about 65–78% to soil CO2 in each soil gas sampling layer. In addition, the humus-derived CO2 contributes ∼81% on average to total soil CO2 in the profile, in good agreement with the field observation. The distribution and origin of soil 14CO2 imply that soil CO2 will be an important source of atmospheric 14CO2 well into the future.

Soil respiration and human effects on global grasslands

Grasslands comprise approximately 40% of the earth's land area (excluding areas of permanent ice cover) and play a critical role in the global carbon cycle. In this paper, by reviewing literature, we quantify annual soil CO 2 efflux, contribution of root respiration to total soil respiration, apparent temperature sensitivity of soil respiration (indicated by Q 10), and turnover rates of soil organic carbon (SOC). We discuss effects of human activities (grazing, land-use changes, and fertilization) on soil respiration rates of global natural grasslands. The soil CO 2 efflux from temperate and tropical natural grasslands is 389.8 ± 45.5 g C m − 2 yr − 1 and 601.3 ± 45.6 g C m − 2 yr − 1 (mean ± S.E.), respectively. The contribution of root respiration to total soil respiration averages 36%, ranging from 8% to 64%. Annual soil CO 2 efflux increases with temperature and precipitation, but increased precipitation can cause a decrease in soil respiration rate in rainy regions. Mean turnover rates of SOC are 71 years in temperate grasslands and 15 years in tropical grasslands. The average Q 10 value is 2.13, with 2.23 for temperate grasslands and 1.94 for tropical grasslands. Human activities significantly affect soil respiration but the extent varies among sites.

Simulation of Soil Organic Carbon Dynamics in Chinese Rice Paddies from 1980 to 2000

Changes in soil organic carbon (SOC) of rice paddies in China were simulated from 1980 to 2000 by linking a coupled bio-physical model to GIS database. The coupled model consists of two sub-models including Crop-C for simulating net primary productivity and hence residue retention and Soil-C for computing the turnover rates of SOC. The GIS database included parameters of climate, soils and agricultural activities with the resolution of 10 km × 10 km. Model simulation indicated that Chinese rice paddies covering 22.6 Mha sequestrated a considerable amount of C, about 0.15 ± 0.07 Pg C from 1980 to 2000. Annual sequestration rate increased sharply from −180 ± 45 kg C ha −1 year −1 in 1980 to 440 ± 170 kg C ha −1 year −1 in 1989. Thereafter, a steady sequestration rate of 460 ± 170 kg C ha −1 year −1 occurred till 1994 and declined since then. Approximately 84% of the Chinese rice paddies sequestrated carbon, while 15% lost carbon and 1% kept balance over the 20 years. Great SOC sequestration occurred in eastern, southern and central China, while a slight decline of SOC existed in some regions of northeastern and southwestern China.

Influence of Fertilizers Applied to a Paddy−Upland Rotation on Characteristics of Soil Organic Carbon and Humic Acids

The qualitative and quantitative characteristics of soil organic carbon (SOC) and related humic acids (HAs) extracted from the soils of field plots were investigated after 8 years of annual paddy (Oryza sativa L.) and upland maize (Zea mays L.) rotation with various fertilizations. Seven fertilization treatments were selected: Ck (no inputs); Chem (chemical fertilizer of NPK); Comp (swine compost); Comp + 33% of Chem N rate; Comp + 67% of Chem N rate; GM (legume green manure) + 33% of Chem N rate; and peat + 33% of Chem N rate. Organic and inorganic nitrogen inputs of six treatments were equivalent with respect of nitrogen content, but Comp, GM, and peat treatments were complemented with various amounts of inorganic N. After harvest of the eighth paddy crop, surface soil samples collected from the plots were subjected to soil characterizations and extraction of humic substances, which were used for chemical, spectroscopic (FTIR, 13C NMR, ESR, X-ray diffractometry), delta13C, and 14C dating analyses. The yields of HAs extracted from the seven treatments were significantly different. Treatment containing persistent organic compound such as the peat + 33% N treatment increased the humification process in topsoils and produced higher yield of HA. Spectroscopic analyses revealed that fertilization treatments changed the functional groups, alkyl C, crystalline characteristics, and delta13C ratios of HAs and turnover rate of SOC considerably. The SOC of the peat + 33% N treatment had the highest mean residence time of 3100 years. Various fertilizer treatments are correlated with turnover rate of SOC and related HAs, which are associated with concerned carbon sequestration as well as mitigation of CO2 emission in the soil environment.

Organic matter turnover in soil physical fractions following woody plant invasion of grassland: Evidence from natural 13C and 15N

Soil physical structure causes differential accessibility of soil organic carbon (SOC) to decomposer organisms and is an important determinant of SOC storage and turnover. Techniques for physical fractionation of soil organic matter in conjunction with isotopic analyses (δ 13C, δ 15N) of those soil fractions have been used previously to (a) determine where organic C is stored relative to aggregate structure, (b) identify sources of SOC, (c) quantify turnover rates of SOC in specific soil fractions, and (d) evaluate organic matter quality. We used these two complementary approaches to characterize soil C storage and dynamics in the Rio Grande Plains of southern Texas where C 3 trees/shrubs (δ 13C=−27‰) have largely replaced C 4 grasslands (δ 13C=−14‰) over the past 100–200 years. Using a chronosequence approach, soils were collected from remnant grasslands (Time 0) and from woody plant stands ranging in age from 10 to 130 years. We separated soil organic matter into specific size/density fractions and determined their C and N concentrations and natural δ 13C and δ 15N values. Mean residence times (MRTs) of soil fractions were calculated based on changes in their δ 13C with time after woody encroachment. The shortest MRTs (average=30 years) were associated with all particulate organic matter (POM) fractions not protected within aggregates. Fine POM (53–250 μm) within macro- and microaggregates was relatively more protected from decay, with an average MRT of 60 years. All silt+clay fractions had the longest MRTs (average=360 years) regardless of whether they were found inside or outside of aggregate structure. δ 15N values of soil physical fractions were positively correlated with MRTs of the same fractions, suggesting that higher δ 15N values reflect an increased degree of humification. Increased soil C and N pools in wooded areas were due to both the retention of older C 4-derived organic matter by protection within microaggregates and association with silt+clay, and the accumulation of new C 3-derived organic matter in macroaggregates and POM fractions.