Hot Water Extractable Organic C Research Articles

Beyond conventional farming: exploring the benefits of planting basins with manure on soil quality as reflected in labile organic carbon and nitrogen indicators in Kenya

PurposeSoil quality is critical for plant growth and ecosystem sustainability. Understanding the indicators that indicate soil quality is, therefore, crucial. Labile organic carbon (C) and nitrogen (N) are important components of soil functions that impact productivity and environmental stability. This study aimed to explore the sensitivity of different labile C and N fractions, including water extractable organic C (WEOC) and total N (WEON) and hot water extractable organic C (HWEOC) and total N (HWEON), to planting basins with manure compared to conventional farming practices.Methods and methodsSoil samples were drawn from 0–10 and 10–20 cm soil depths at 12 sites in Makueni County, Kenya. Samples were analysed for differences in WEOC, HWEOC, WEON, and HWEON between planting basins with manure and conventional farmer practices. We also assessed the correlations of the different labile C and N pools with other soil chemical properties linked to soil quality.Results and discussionsThe results showed that planting basins with manure significantly increased mean WEOC (171.53 µg g−1), HWEOC (353.62 µg g−1), WEON (26.60 µg g−1), and HWEON (26.39 µg g−1) compared to those of conventional farming practices (p < 0.05). WEOC was positively correlated with WEON and δ15N (p < 0.001) at the 0–10 cm soil layer suggesting that extractable organic matter can be used as an index for soil health and nutrient content.ConclusionThe findings suggest that cold water and hot water extractable C and N are sensitive indicators of the effects of different land management practices on soil quality. The results further demonstrate that planting basins with manure can improve soil quality by increasing labile C and N pools.

Linking between soil properties, bacterial communities, enzyme activities, and soil organic carbon mineralization under ecological restoration in an alpine degraded grassland.

Soil organic carbon (SOC) mineralization is affected by ecological restoration and plays an important role in the soil C cycle. However, the mechanism of ecological restoration on SOC mineralization remains unclear. Here, we collected soils from the degraded grassland that have undergone 14 years of ecological restoration by planting shrubs with Salix cupularis alone (SA) and, planting shrubs with Salix cupularis plus planting mixed grasses (SG), with the extremely degraded grassland underwent natural restoration as control (CK). We aimed to investigate the effect of ecological restoration on SOC mineralization at different soil depths, and to address the relative importance of biotic and abiotic drivers of SOC mineralization. Our results documented the statistically significant impacts of restoration mode and its interaction with soil depth on SOC mineralization. Compared with CK, the SA and SG increased the cumulative SOC mineralization but decreased C mineralization efficiency at the 0-20 and 20-40 cm soil depths. Random Forest analyses showed that soil depth, microbial biomass C (MBC), hot-water extractable organic C (HWEOC), and bacterial community composition were important indicators that predicted SOC mineralization. Structural equal modeling indicated that MBC, SOC, and C-cycling enzymes had positive effects on SOC mineralization. Bacterial community composition regulated SOC mineralization via controlling microbial biomass production and C-cycling enzyme activities. Overall, our study provides insights into soil biotic and abiotic factors in association with SOC mineralization, and contributes to understanding the effect and mechanism of ecological restoration on SOC mineralization in a degraded grassland in an alpine region.

Water extractable organic C and total N: The most sensitive indicator of soil labile C and N pools in response to the prescribed burning in a suburban natural forest of subtropical Australia

Soil labile carbon (C) and nitrogen (N) pools are important indicators of soil fertility that are sensitive to environmental disturbance. However, few studies have periodically examined the long-term dynamics of soil labile C and N pools following prescribed burning and their implications on nutrient cycling in forest ecosystems. In this study, 0–5, 5–10, and 10–20 cm soil depths were sampled multiple times, spaning six years across a series of prescribed burning sites in a eucalypt natural forest of subtropical Australia. Water extractable organic C (WEOC) and total N (WETN), hot water extractable organic C (HWEOC) and total N (HWETN), microbial biomass C (MBC) and N (MBN) were analysed and correlated with the post-fire recovery time and other major environmental factors. Prescribed burning had long-lasting impact on WEOC and WETN for nearly 12 years following the burning. The lengthy recovery time was associated with the slow recovery in the forest floor of organic layers, which significantly diminished upon the burning. This was longer than the proposed 8–10 year burning cycle in Toohey Forest, and necessitated extending the burning cycle or reducing burning intensity to preserve the soil fertility. The WEOC and WETN were the most sensitive indictors of burning disturbances on soil labile C and N pools in this forest ecosystem. The MBC & MBN, and HWEOC & HWETN, however, did not show consistent patterns of recovery. More studies are needed to differentiate between the burning impacts and the seasonal impacts in the context of intensifying climate change and urban developments.

Effect of grazing exclusion and rotational grazing on labile soil organic carbon in north China

AbstractOvergrazing has caused serious grassland soil deterioration and significant soil organic carbon (SOC) losses. Controlled grazing is recommended for restoring grassland and enhancing SOC stocks. Studies have suggested that, in particular, labile organic carbon (C) is a sensitive indicator for soil degradation and SOC recovery, but few studies of the effect of different grassland management practices have been published so far. Here, we investigate the effects of long‐term grazing exclusion and short‐term rotational grazing with different grazing intensities on labile SOC, including hot water extractable organic C (HWEC) and particulate organic C (POC), as well as potential N mineralization (Nmin) rate in soils (0–10 cm) of two grasslands in Xilinhot, Inner Mongolia. Treatments include long‐term (34 years) exclusion, medium‐term (21 years) exclusion and continuous grazing (ambient grazing) at one of the sites, and we investigate no grazing, light grazing, moderate grazing and high‐grazing intensity at the study site for rotational grazing. Our results show that grazing exclusion significantly enhances labile SOC stocks and potential Nmin rate compared to ambient grazing (e.g., POC stock at long‐ and medium‐term exclusion and ambient grazing are 5.71 ± 0.94 Mg C ha−1, 3.25 ± 0.30 Mg C ha−1 and 2.12 ± 0.38 Mg C ha−1, respectively). However, after two decades of exclusion, the potential for further C sequestration is limited. Five years of rotational grazing significantly enhances HWEC, but not POC and potential Nmin rate, when comparing the highest grazing intensity (1.88 sheep ha−1 yr−1) with grazing exclusion during the same period. We conclude that rotational grazing may be a viable alternative to exclusion, offering a more rational use of grassland without causing soil C loss. We also show that HWEC is a particularly sensitive labile SOC indicator and is tightly correlated to POC and potential Nmin rate.Highlights We investigated effects of grazing exclusion and rotational grazing on labile soil organic carbon. Grazing exclusion increased labile SOC and the highest level appeared in medium‐term exclusion. Five years of high‐intensity rotational grazing increased HWEC with no significant effects on POC. Rotational grazing is a viable alternative to grazing exclusion management.

The development of soil organic carbon under young black locust (Robinia pseudoacacia L.) trees at a post-mining landscape in eastern Germany

The aim of this study was to evaluate the potential of short rotation alley cropping systems (SRACS) to improve the soil fertility of marginal post-mining sites in Brandenburg, Germany. Therefore, we annually investigated the crop alleys (AC) and black locust hedgerows (ABL) of a SRACS field trail under initial soil conditions to identify the short-term effects of tree planting on the storage of soil organic carbon (SOC) and its degree of stabilization by density fractionation. We detected a significant increase in SOC and hot-water-extractable organic C (HWEOC) at ABL, which was mainly restricted to the uppermost soil layer (0–10 cm). After 6 years, the SOC and HWEOC accumulation rates at ABL were 0.6 Mg and 46 kg ha−1 year−1, which were higher than those in the AC. In addition, comparatively high stocks of approximately 4.6 Mg OC and 182 kg HWEOC ha−1 were stored in the ABL litter layer. Density fractionation of the 0–3 cm soil layer at ABL revealed that the majority of the total SOC (47%) was stored in the free particulate organic matter fraction, which was more than twice that of the AC. At the same time, a higher and steadily increasing amount of SOC was stored in the occluded particulate organic matter fraction at ABL, which indicated a high efficiency for SOC stabilization. Overall, our findings support the suitability of black locust trees for increasing the soil fertility of the reclaimed mining substrate and, consequently, the high potential for SRACS to serve as an effective recultivation measure at marginal sites.

Residue retention mitigated short-term adverse effect of clear-cutting on soil carbon and nitrogen dynamics in subtropical Australia

This study aimed to investigate the benefits of retaining harvest residues on the dynamics of soil C and N pools following clear-cut harvesting of a slash pine plantation in South East Queensland of subtropical Australia. Immediately following clear-cut harvesting, macro-plots (10 × 10 m) were established on a section of the plantation in a randomised complete block design with four blocks and three treatments: (1) residue removal (RR0), (2) single level of residue retention (RR1) and (3) double level of residue retention (RR2). Soils were sampled at 0, 6, 12, 18 and 24 months following clear-cutting and analysed for total C and N, microbial biomass C (MBC) and N (MBN), hot water–extractable organic C (HWEOC), hot water–extractable organic N (HWEON), NH4+–N and NOx−–N. The study showed that although soil total C decreased in the first 12 months following clear-cutting, harvest residue retention increased soil total C and N by 45% (p < 0.001) and 32% (p < 0.001), respectively, over the 12–24 months. NH4+–N, HWEOC, HWEON and MBC showed initial surges in the first 6 months irrespective of residue management, which declined after the 6th month. However, residue retention significantly increased HWEOC and HWEON over the 12–24 months (p < 0.001). This study demonstrated that harvest residue retention during the inter-rotation period can minimise large changes in C and nutrient pools, and can even increase soil C and nutrient pools for the next plantation rotation.

Effects of changed litter inputs on soil labile carbon and nitrogen pools in a eucalyptus-dominated forest of southeast Queensland, Australia

The quantity and quality of litter inputs to forest soils are likely to be changed as a result of the climate change and human disturbances. However, the effects of changed litter inputs on soil labile carbon (C) and nitrogen (N) pools still remain unclear. A 15-month in situ field experiment was conducted within both high and low litter quality site in a eucalyptus-dominated native forest of Queensland, Australia. Three rates of litter inputs were applied, including (i) no litter (NL); (ii) single litter (SL), representing the average condition of the surrounding forest floor; and (iii) double litter (DL). Water-extractable organic C (WEOC) and total N (WETN), hot water-extractable organic C (HWEOC) and total N (HWETN), microbial biomass C (MBC), and N (MBN) were analyzed in the 0–5-cm soil layer seasonally. Litter input rates had no significant effects on litter decomposition at both sites (P > 0.05). After 15-month of decomposition, mean litter mass loss was 46.3% and 31.2% at the HQ and LQ sites, respectively. Changed litter quantity had no significant effects on any of the soil labile C and N pools, regardless of litter quality. However, soil labile C and N pools significantly varied with sampling times, and the samples of different sampling times were clearly separated at both sites according to the redundancy analysis (RDA). WEOC peaked in summer, declined in autumn and winter, and increased again in spring, while the concentrations of HWEOC and MBC peaked in the winter period. The seasonal trends of MBN were opposite to the trends of WETN, which might be due to the temporal partitioning of N between plants and microbes. The findings indicated that soil labile C and N pools in the eucalyptus-dominated forest of subtropical Australia were resistant to a short-term change in aboveground litter inputs. Future research should expand on these findings by keeping observing over a longer time period and considering the influence of changed belowground litter inputs.

Subsoil application of compost improved sugarcane yield through enhanced supply and cycling of soil labile organic carbon and nitrogen in an acidic soil at tropical Australia

Organic amendments are mostly carried out on the soil surface layer to mitigate the decline in sugarcane soil health, however, the effects of subsoil application of composts on soil carbon (C) and nutrient dynamics and sugarcane yield are largely unknown. A 4 years field trial was conducted at Maryborough, Australia, to investigate the effects of subsoil application of compost on soil C and N cycling, associated biological processes and sugarcane productivity. The trial included four subsoil (ca. 25 cm) amendment treatments of control (CK, without amendment); gypsum (GP); compost (CP); and mineral fertilizer (FE). Overall, the compost treatment increased concentrations of soil NH4+-N by 30% and NO3−-N by 40% at subsoil (10–25 cm) amendment layer. Soil microbial biomass C and N at the 0–10 cm depth were also significantly higher in the compost treatment than the other treatments. The CO2 respiration in the compost and fertilizer treatments, were significantly higher than in the control and gypsum treatments at the subsoil amendment depth. The compost treatment had greater β–glucosidase activities than other treatments at the 10–25 cm soil depth. In addition, subsoil application of compost increased inputs of hot water extractable organic C (HWEOC) by 30% and N (HWETN) by 70% at the application depth, but did not affect total soil C and N contents. The HWEOC and HWETN pools were positively related to the cumulative sugarcane yield during the 4 years cropping cycle. The subsoil application of compost increased the supply of organic C and N for microbial community, enhanced nutrient cycling processes, improved soil environmental conditions and soil health for sugarcane growth and thus increased sugarcane productivity.

Enzymes and C pools as indicators of C build up in short-term conservation agriculture in a savanna ecosystem in Cambodia

Soil organic carbon (SOC) pools, particularly labile pools, and soil enzymes are good indicators of short-term impacts of soil management. We hypothesized that labile SOC pools drives C accumulation and enzyme activity can be an efficient indicator for C build up in short-term conservation agriculture. The aims of this study were to quantify the impacts of tillage and crop rotations with diverse crop residue inputs on changes in SOC labile pools and enzymatic activities in rice-, soybean- and cassava-based cropping systems designated as RcCS, SbCS and CsCS, respectively. The four treatments in each cropping system consisted of: conventional tillage (CT), no-till in which main crops (rice, soybean and cassava) were grown in a one-year frequency pattern (NT1) and, no-till in which the main crops were grown in bi-annual rotations with maize (NT2 and NT3). After 5 years experiment period, greater hot-water extractable organic C (HWEO-C) stocks of 61%, 55% and 53%, and permanganate oxidizable C (POX-C) stocks of 23%, 21% and 32% were attributed to NT than those in CT soils under RcCS, SbCS and CsCS, respectively, at 0–5 cm soil layer. The pyrophosphate extractable organic C (PEO-C) and chemically stabilized organic C (CSO-C) stocks were almost constant in each depth among treatments, except 0–5 cm in CsCS. The β-glucosidase activity was 18%, 28% and 49% greater in NT than those in CT soils at 0–5 cm under RcCS, SbCS, CsCS, respectively. Arylsulfatase activity was 36% and 39% greater in NT than in CT under SbCS and CsCS, respectively but no significant differences in RcCS. A strong and positive correlation (P < 0.001) between β-glucosidase and arylsulfatase activity with POX-C, HWE-C, SOC and total N indicated how these variables were inter-related. Comparison among three NT treatments, bi-annual crop rotations showed a better increasing trend of HWEO-C, POX-C and enzymatic activities than those with one-year frequency pattern. In conclusion, short-term NT crop rotations with permanent soil cover significantly increased the storage of HWEO-C and POX-C and enhanced β-glucosidase and arylsulfatase activities at the surface soil layer as a result of higher biomass-C input and the absence of soil disturbance.

Interactive effects of biochar addition and elevated carbon dioxide concentration on soil carbon and nitrogen pools in mine spoil

This study aimed to assess the effects of biochar on improving nitrogen (N) pools in mine spoil and examine the effects of elevated CO2 on soil carbon (C) storage. The experiment consisted of three plant species (Austrostipa ramossissima, Dichelachne micrantha, and Lomandra longifolia) planted in the N-poor mine spoil with application of biochar produced at three temperatures (650, 750, and 850 °C) under both ambient (400 μL L−1) and elevated (700 μL L−1) CO2. We assessed mine spoil total C and N concentrations and stable C and N isotope compositions (δ13C and δ15N), as well as hot water extractable organic C (HWEOC) and total N (HWETN) concentrations. Soil total N significantly increased following biochar application across all species. Elevated CO2 induced soil C loss for A. ramossissima and D. micrantha without biochar application and D. micrantha with the application of biochar produced at 750 °C. In contrast, elevated CO2 exhibited no significant effect on soil total C for A. littoralis, D. micrantha, or L. longifolia under any other biochar treatments. Biochar application is a promising means to improve N retention and thus, reduce environmentally harmful N fluxes in mine spoil. However, elevated CO2 exhibited no significant effects on increasing soil total C, which indicated that mine spoil has limited potential to store rising atmospheric CO2.

Biochar nutrient availability rather than its water holding capacity governs the growth of both C3 and C4 plants

Biochar has been suggested as a soil conditioner to improve soil fertility and crop productivity while simultaneously mitigate global climate change by storing carbon in the soil. This study investigated the effect of pine (Pinus radiata) biochar application on soil water availability, nitrogen (N) and carbon (C) pools and growth of C3 and C4 plants. In a glasshouse pot trial, a pine biochar (untreated) and nutrient-enriched pine biochar were applied to a market garden soil with C3 (Spinacia oleracea L.) and C4 (Amaranthus paniculatus L.) plants at rates of 0, 1.0, 2.0, and 4.0 % (w/w). Plant biomass, soil pH, moisture content, water holding capacity (WHC), hot water extractable organic C (HWEOC), and total N (HWETN), total C and N, and their isotope compositions (δ 13C and δ 15N) of soils and plants were measured at the end of the experimentation. The soil moisture content increased while plant biomass decreased with increasing untreated biochar application rates. The addition of nutrient-enriched biochar significantly improved plant biomass in comparison to the untreated biochar addition at most application rates. Biochar application also increased the levels of labile organic C and N pools as indicated by HWEOC and HWETN. The results suggested that the addition of pine biochar significantly improved soil water availability but not plant growth. The application of nutrient-enriched pine biochar demonstrated that the growth of C3 and C4 plants was governed by biochar nutrient availability rather than its water holding capacity under the pot trial condition.

Soil carbon and nitrogen dynamics in the first year following herbicide and scalping in a revegetation trial in south-east Queensland, Australia

During revegetation, the maintenance of soil carbon (C) pools and nitrogen (N) availability is considered essential for soil fertility and this study aimed to evaluate contrasting methods of site preparation (herbicide and scalping) with respect to the effects on soil organic matter (SOM) during the critical early establishment phase. Soil total C (TC), total N (TN), hot-water extractable organic C (HWEOC), hot-water extractable total N (HWETN), microbial biomass C and N (MBC and MBN), total inorganic N (TIN) and potentially mineralizable N (PMN) were measured over 53 weeks. MBC and MBN were the only variables affected by herbicide application. Scalping caused an immediate reduction in all variables, and the values remained low without any sign of recovery for the period of the study. The impact of scalping on HWETN and TIN lasted 22 weeks and stabilised afterwards. MBC and MBN were affected by both herbicide and scalping after initial treatment application and remained lower than control during the period of the study but did not decrease over time. While scalping had an inevitable impact on all soil properties that were measured, that impact did not worsen over time, and actually improved plant growth (unpublished data) while reducing site establishment costs. Therefore, it provides a useful alternative for weed control in revegetation projects where it is applied only once at site establishment and where SOM would be expected to recover as canopy closure is obtained and nutrient cycling through litterfall commences.

The impact of mulch type on soil organic carbon and nitrogen pools in a sloping site

Three mulch treatments were tested for their ability to control erosion on a sloping site. Additionally, the choice of mulch can also enhance revegetation success and improve soil organic matter input. This study aimed to investigate the effects of three mulching treatments, hydro-seeding, granite mulch and forest mulch, on soil C and N pools at different positions on highly erodible slope with approximately 30 % gradient. Soil moisture, total C (TC), total N (TN), hot water-extractable organic C (HWEOC), hot water-extractable total N (HWETN), microbial biomass C and N (MBC and MBN), inorganic N and potentially mineralisable N were measured. All variables were significantly higher in soils amended with forest mulch than those with hydro-seeding and granite mulch, for the same slope positions. Soil moisture was significantly higher in the lower slope position than middle and upper slope positions in hydro-seeding and granite mulch treatments, whereas no slope effect was observed on soil moisture under the forest mulch application. In the forest mulch treatment, the upper slope position had higher soil TC, TN, HWEOC, HWETN, MBC, MBN, NO3−-N and total inorganic N than the middle and lower slope positions. Five years following mulch application, forest mulch still exerted a significant influence on soil fertility compared to the other treatments and the influence on soil moisture suggests that this treatment would be the most effective in the control of water-driven soil erosion on this steep site.

Aggregate C depletion by plowing and its restoration by diverse biomass-C inputs under no-till in sub-tropical and tropical regions of Brazil

Encapsulation of soil organic carbon (SOC) within aggregates is one of the principal mechanisms for long-term C sequestration, macroaggregate formation and stabilization. Our objectives were to quantify the changes in aggregate size distribution, aggregate-C concentrations and stocks upon conversion of native vegetation (NV) to conventional plow-based tillage (CT), and to assess the rate of aggregation and SOC recovery with no-till (NT) under diverse biomass-C inputs. The study was conducted at both sub-tropical (Ponta Grossa – PG, State of Paraná) and tropical (Lucas do Rio Verde – LRV, State of Mato Grosso) sites in Brazil. The SOC content under NV was used as a baseline to evaluate the depletion rate under CT and the restoration rate under NT. A specific emphasis was given to the largest macroaggregate size class (8–19mm) because of its importance to protecting the recently deposited labile SOC. A discriminant analysis of principal components (DAPC) indicated that NV soil is modified by conversion to an arable land use and that, mechanical tillage, biomass input, and their interactions drastically influence the distribution of aggregate-size classes, aggregation indices, and SOC distribution within aggregates. At both sites, soil aggregation indices were positively impacted by NT and associated with SOC concentration in the labile fractions (e.g., total polysaccharides (TPS), hot water extractable organic C (HWEOC), particulate organic C (POC)). At the PG site, the 8–19mm aggregate size fraction was significantly affected by land use and tillage treatments and represented 54%, 43%, and 72%, under NV, CT, and NT in 0–20cm depth, respectively. Furthermore, the 8–19mm size fraction stored 55%, 45%, and 71% of the total SOC stock under NV (53.8MgCha−1), CT (28.5MgCha−1) and NT (51.2MgCha−1), respectively. At the LRV site, the 8–19mm aggregate size fraction decreased from 50% under Cerrado NV to 35% under CT, and ranged from 33% to 51% under diverse biomass-C input under NT in 0–20cm depth. The 8–19mm size fraction stored 52%, 37%, and 41% of the total SOC stock across all aggregate sizes under NV (25.4MgCha−1), CT (11.7MgCha−1), and NT (9.9–18.1MgCha−1), respectively. The difference in SOC stock among land uses is largely attributed to storage in the 8–19mm aggregate size class, indicating that NT cropping systems rebuilt the largest macroaggregates, which are crucial for stabilization of SOC.

Effects of pre-planting site management on soil organic matter and microbial community functional diversity in subtropical Australia

Weed control is a key factor affecting early plant growth and establishment in revegetation projects in South-east Queensland of sub-tropical Australia. Costs associated with weed control are significant and methods which reduce establishment costs and effectively suppress weeds are of great interest. However, different methods may have implications for soil quality and fertility and require a detailed investigation. Understanding the response of soil organic matter (SOM) and microbial functional diversity to different weed control methods is crucial as they affect soil quality and nutrient availability. A field trial was established in South-east Queensland to identify the effects of three methods of weed control: (1) glyphosate, (2) a mixture of glyphosate and MCPA and (3) topsoil removal or scalping on SOM, microbial biomass, soil respiration, NH4+-N availability, potentially mineralisable N (PMN) and soil microbial community functional diversity (as assessed by carbon substrate utilisation using Biolog GN2 plates). The scalped area had lower SOM and microbial activity compared to the herbicide and control plots. There was no significant difference in water soluble organic carbon (WSOC), hot water extractable organic C (HWEOC), hot water extractable total N (HWETN) and microbial biomass C and N (MBC and MBN) between the herbicide and control plots, particularly at week 20. NH4+-N and PMN values were lower at week 20 than week 1 in the herbicide and scalping treatments. Week 20 was the end of the growing season and reduction in N availability may have been the result of decreasing temperature. Principal component analyses (PCA) from Biolog GN2 results indicated a separation in soil microbial community function in the scalped area compared to the other treatments which may have implications for soil properties in the long term.Scalping proved to be the most cost-effective method of site preparation, requiring fewer site visits for weed control compared to herbicide application. However, SOM was significantly affected by scalping due to topsoil removal and it may not be a sustainable practice in short rotation plantation establishment. Single herbicide application at field rates did not impact soil organisms but also failed to achieve proper weed control.

Soil soluble carbon dynamics of manured and unmanured grasslands following chemical kill and ploughing

Better understanding of soluble carbon (C) dynamics in managed grasslands fertilised with animal manures is required to effectively estimate nutrient transport potential and C loss during land-use change. Two adjacent grassland plots, one amended with 100 m 3 ha − 1 of liquid swine manure annually since 1978 and another unamended for the same period, were either killed by glyphosate in the autumn or left with vegetation intact. Those killed were either (i) left as an undisturbed chemical fallow, (ii) ploughed (20 cm depth) by full inversion tillage (FIT) in the autumn, or (iii) ploughed (20 cm depth) in the following spring. Microbial biomass C (MBC) and soluble C fractions: hot-water extractable organic C (HWEOC), cold-water extractable organic C (CWEOC), and dissolved organic C from tension lysimeters (DOC) were monitored during one full year following herbicide application and ploughing. On both unmanured and manured sites, HWEOC was positively correlated with MBC. However, on the manured grassland the slope of the HWEOC:MBC relationship was 55% higher than on the unmanured site indicating that the manured grassland accumulated more potentially soluble C per unit MBC than the unmanured grassland. Compared to the plots with intact vegetation and the chemical fallow, full inversion tillage reduced HWEOC and CWEOC by 10 to 30%, on both manured and unmanured soils, and reduced DOC in the soil solution of the manured soils. Peak DOC concentrations measured in the field occurred when soils were water-logged, particularly on the manured grassland. Our results suggest that manured grasslands on poorly-drained soils accumulate important quantities of readily soluble C which are prone to export as DOC when the soil becomes water-saturated. However, full inversion tillage of these grassland soils attenuated the amounts of readily soluble C and, in particular, the peaks in DOC.

Biological, chemical and thermal indices of soil organic matter stability in four grassland soils

Abstract The various ecosystem functions of soil organic matter (SOM) depend on both its quantity and stability. Numerous fractionation techniques have been developed to characterize SOM stability, and thermal analysis techniques have shown promising results to describe the complete continuum of SOM in whole soil samples. However, the potential link between SOM thermal stability and biological or chemical stability has not yet been adequately explored. The objective of this study was to compare conventional chemical and biological methods used to characterize SOM stability with results obtained by thermal analysis techniques. Surface soil samples were collected from four North American grassland sites along a continental mean annual temperature gradient, each with a native and cultivated land use. Soil organic C concentrations ranged from 6.8 to 33 g C kg −1 soil. Soils were incubated for 588 days at 35 °C, and C mineralization rates were determined periodically throughout the incubation by measuring CO 2 concentration using an infrared gas analyzer (IRGA) to calculate biological indices of SOM stability. Hot-water extractable organic C (HWEOC) contents were determined before and after incubation as chemical indices. Finally, samples from before and after incubation were analyzed by simultaneous thermal analysis (i.e., thermogravimetry (TG) and differential scanning calorimetry (DSC)) to determine thermal indices of SOM stability. Long-term incubation resulted in the mineralization of up to 33% of initial soil C. The number of days required to respire 5% of initial soil organic carbon (SOC), ranged from 27 to 115 days, and is proposed as a standardized biological index of SOM stability. The number of days was greater for cultivated soils compared to soils under native vegetation, and generally decreased with increasing site mean annual temperature. HWEOC (as % of initial SOC) did not show consistent responses to land use, but was significantly lower after long-term incubation. Energy density (J mg −1 OM) was greater for soils under native vegetation compared to cultivated soils, and long-term incubation also decreased energy density. The temperatures at which half of the mass loss or energy release occurred typically showed larger responses to land use change than to incubation. Strong correlations demonstrated a link between the thermal and biogeochemical stability of SOM, but the interpretation of the thermal behavior of SOM in bulk soil samples remains equivocal because of the role the mineral component and organo-mineral interactions.

Changes in soil carbon during the establishment of a hardwood plantation in subtropical Australia

Soil carbon (C) pools are not only important to governing soil properties and nutrient cycling in forest ecosystems, but also play a critical role in global C cycling. Mulch and weed control treatments may alter soil C pools by changing organic matter inputs to the forest ecosystem. We studied the 12-month mulch and weed control responses on the chemical composition of soil organic C and the seasonal dynamics of water extractable organic C (WEOC), hot water extractable organic C (HWEOC), chloroform-released organic C (CHCl3-released C), and acid hydrolysed organic C (acid hydrolysable C) in a hardwood plantation of subtropical Australia. The results showed that compared with the non-mulch treatment, the mulch treatment significantly increased soil WEOC, HWEOC, and CHCl3-released C over the four sampling months. The weed control treatment significantly reduced the amount of HWEOC and CHCl3-released C compared with the no weed control treatment. Neither the mulch nor weed control treatment significantly affected soil acid hydrolysed organic C. There were no significant seasonal variations in soil WEOC, HWEOC, CHCl3-released C, and acid hydrolysed organic C in the hardwood plantation. Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy was used to study the structural chemistry of soil C pools in hydrofluoric acid (HF) treated soils collected 12 months after the mulch and weed control treatments were applied. Overall, O-alkyl C was the dominant C fraction, accounting for 33–43% of the total NMR signal intensity. The mulch treatment led to higher signal intensity in the alkyl C spectral region and A/O-A ratio (the ratio of alkyl C region intensity to O-alkyl C region intensity), but lower signal intensity in the aryl C and aromaticity. Compared with the no weed control treatment, the weed control treatment reduced signal intensity in the aryl C and aromaticity. Together, shifts in the amount and nature of soil C following the mulch and weed control treatments may be due to the changes in organic matter input and soil physical environment.

Soil carbon and nitrogen pools and microbial properties in a 6-year-old slash pine plantation of subtropical Australia: impacts of harvest residue management

Harvest residue management can affect the dynamics of soil carbon (C) and nutrient pools and associated soil microbial processes. A field experiment was conducted to investigate the impacts of harvest residue management practices on soil C and nitrogen (N) pools in a slash pine plantation grown on a sandy soil of subtropical Australia. Results showed that harvest residue retention significantly enhanced accumulation of soil total C and N compared with residue removal. The NH 4 +-N was the predominant form of soil mineral N, and there were no significant impacts of residue management practices on concentrations of soil NH 4 +-N measured at the time of sampling. Concentrations of water-soluble and hot water extractable organic C and total N tended to be higher in soil with residue retention compared with residue removal, but this trend was only significant for hot water extractable organic C (HWEOC) in surface soil (0–10 cm). Residue retention also tended to increase soil microbial biomass C and N, but did not significantly affect soil respiration and metabolic quotient ( qCO 2). Direct C and N inputs into soil from the residue layer and moderation of the variation of soil moisture and temperature over the seasons by harvest residue cover might have contributed to the accumulation of soil C and N and microbial biomass with the residue retention treatments. The lack of statistically significant differences in some of soil microbial properties (e.g. microbial biomass C, respiration, etc.) might have been related to a large spatial variability among replicate plots at the experimental site.

Soil Carbon Pools in Adjacent Natural and Plantation Forests of Subtropical Australia

Soil C dynamics are not only important to both productivity and sustainability of terrestrial ecosystems, but also contribute significantly to global C cycling. Adjacent natural forest (NF), and first (1R) and second rotation (2R) hoop pine (Araucaria cunninghamii Aiton ex A. Cunn.) plantations in southeast Queensland, Australia, were selected to investigate the effects of conversion of NF to hoop pine plantations and forest management (harvesting and site preparation of plantation) on the size and the nature of C pools in surface (0–10 cm) soils using chemical extraction, laboratory incubation and 13C cross‐polarization with magic‐angle‐spinning nuclear magnetic resonance spectroscopy (13C CPMAS NMR). Conversion from NF to hoop pine plantations not only led to the reduction of soil total C (by 19.8%), water‐soluble organic C (WSOC) (by 17.7%), CaCl2–extractable organic C (by 38.8%), and hot water‐extractable organic C (HWEOC) (by 30.9%) and bioavailability of soil C (as determined by CO2 evolved in the incubation), but also to a change in chemical composition of soil C with lower O‐alkyl C and higher alkyl C under the 1R plantation compared with NF. Harvesting and site preparation did not significantly affect total soil C and most labile C pools (except for a decrease in WSOC), but led to a lower signal intensity in the alkyl C spectral region and a decreased alkyl C/O‐alkyl C (A/O‐A) ratio in the soil under the 2R compared with the 1R plantation. The shifts in the amount and nature of soil C following forest conversion may be attributed to changes in litter inputs, microbial diversity and activity, and the disturbance of soil during harvesting and site preparation.