Aggregate-associated Soil Organic Carbon Research Articles

Effects of rehabilitation through afforestation on soil aggregate stability and aggregate-associated carbon after forest fires in subtropical China

The recovery of carbon stocks through the afforestation of degraded lands is a management practice that can be applied to mitigate rising concentrations of atmospheric carbon dioxide; however, due to economic pressure, most plantations are cut down in the growth stage, and the effect of forest stand age on soil carbon accumulation is unknown. We evaluated the effects of forest stand age on soil organic carbon (SOC), soil aggregate stability, and aggregate-associated SOC during afforestation. We collected soils along a chronosequence of 10, 20, 40, 60, and 100 years following restoration with P. massoniana after forest fires in the subtropical region of China. The results showed that the soil aggregates were primarily macroaggregates (>10 mm, 2–5 mm), and the stability of the soil aggregates first increased and then decreased as time progressed. The stability of the soil aggregates was at its maximum at 20 years post-planting. The mean weight diameter (MWD), geometric mean diameter (GMD) and fractal dimension (D) were 5.90 ± 0.17 mm, 4.25 ± 0.21 mm and 2.10 ± 0.04 mm, respectively, for the soil aggregates in the 0–20 cm layer. SOC content continuously increased with time since restoration, and SOC storage was mainly concentrated in the soil aggregates with sizes of >10 mm and 2–5 mm. Compared with the forests with restoration times of 10 years, the SOC storage in the 0–20 cm layer in the 20-, 40-, 60-, and 100-year-old forests increased by 10.10%, 9.57%, 31.39% and 69.40%, respectively. Compared with the forests with restoration times of 10 years, the SOC storage in the 80–100 cm layer of 20- and 40-year-old forests did not differ significantly, but those in the 60- and 100-year-old forests increased by 83.76% and 116.85%, respectively. For the forests between 10 and 100 years of age, the soil carbon sequestration rates were 0.33 ± 0.02 (0–20 cm), 0.42 ± 0.07 (20–40 cm), 0.39 ± 0.09 (40–60 cm), 0.33 ± 0.05 (60–80 cm) and 0.28 ± 0.03 (80–100 cm) Mg·C·ha−1·a−1. We concluded that SOC accumulation during afforestation occurs mainly in macroaggregates, which have faster turnover than microaggregates. The soils of mature and over-mature forests still have high carbon storage and sequestration rate, especially in the deeper soil layers (60–80 cm, 80–100 cm).

Relationship Between the Composition of Soil Aggregates and the Distribution of Organic Carbon Under Long-Term Abandoned Restoration

Soil aggregates are important carriers of soil organic carbon (SOC) accumulation, and play an important role in the evaluation of soil structure and quality. Natural recovery can promote change in soil aggregate structure and quantity via the redistribution of SOC in the aggregates. Natural restoration from farmland is an important vegetation restoration model on the Loess Plateau. The changes in soil aggregate structure and soil carbon stock after natural restoration have received extensive attention. However, little is known about the continuous study of soil changes on the abandoned grassland during the recovery process. Therefore, to understand how SOC accumulates in the process of natural recovery and quantitatively analyze the contribution of aggregates to the total soil carbon pool, we selected four abandoned grasslands of different restoration ages on the Loess Plateau, China, and studied the changes in soil structure, soil total organic carbon (TOC), soil C:N, soil aggregate distribution, soil aggregate stable index (mean weight diameter, MWD; geometric mean diameter, GMD), and aggregate-associated SOC changes as well as their correlations from 0-20 cm and 20-40 cm soil layers in abandoned grasslands. In addition, we calculated the contribution of aggregates with different sizes to soil TOC stock. The results showed that:① natural restoration increased the macroaggregate amount, MWD, and GMD, but decreased the amount of microaggregate and silt-and clay-sized fractions. There are significant differences in the distribution and stability of aggregates between different soil layers; the promotion effect of the surface was higher than that of the subsurface soils. ② In the 42 years after abandoning recovery, soil TOC stock, macaggregate-and mesaggregate-associated SOC stock increased significantly, and varied with soil depth and years of abandonment (1.92 times, 10.2 times, and 3.61 times). In contrast, micaggregate-associated SOC stock decreased significantly, and silt-and clay-sized fractions-associated SOC stock showed no distinct change. In addition, natural restoration promoted the ratio of C:N; nevertheless, the ratio of C:N under the surface showed a reduced phenomenon after 42 years of abandonment. ③ The improvement in soil TOC stock depends primarily on changes in the macaggregate-associated organic carbon stocks, which account for 80% of macaggregate, and the significant increase in the amount of macaggregate is the main reason for the high contribution.The results of our study suggest that natural restoration is conducive to the accumulation of soil organic carbon, and improvement in soil structure and stability. Macroaggregate is the key factor in soil organic carbon accumulation and soil structure improvement in the process of natural restoration.

Changes in soil micro- and macro-aggregate associated carbon storage following straw incorporation

Inadequate soil management strategies weaken soil quality. Soil aggregates and aggregate-associated soil organic carbon (SOC) are important attributes of the soil and can serve as indicators of changes in soil quality. The objective of this study was to optimize an improved soil management that slowed turnover of aggregates and increased SOC storage in Northern China. Four tillage methods were examined: (i) RT (rotary tillage without straw return), (ii) RS (rotary tillage with straw return), (iii) DS (deep tillage with straw return), and (iv) TS (RS applied annually and with DS applied at a two-year interval). The results indicated that straw return treatments increased macroaggregate formation and sequestrated greater SOC. The C inputs among RS, DS, and TS were similar in magnitude, ranging from 8.86 to 8.99 t ha−1 year−1. Compared to RS, DS enhanced the turnover of macroaggregates in the 0–20 cm layers, and led to more SOC loss in occluded microaggregates than in macroaggregates in the 0–30 cm layers, indicating a reduced protection of aggregates to SOC. Similar to DS, the mass of macroaggregates under TS was greater than RS in the 10–30 cm layers. However, TS increased the SOC storage in macroaggregates by 0.66 and 0.90 t ha−1 in the 0–30 cm layers, respectively, compared to RS and DS. Moreover, the SOC storage in occluded microaggregates under TS was 0.52 and 1.06 t ha−1 greater than RS and DS, respectively, in the 0–30 cm layers. SOC variations among treatments in this study were dominated by tillage methods; thus, appropriate soil disturbance in deep layers (TS) has great potential in activating more C sequestration in macroaggregates and occluded microaggregates. These findings are of great importance for formulating soil management strategy and probably beneficial for crop production.

Responses of soil aggregate-associated organic carbon and nutrients to tea cultivation age in southern Guangxi, China

Understanding the response mechanism of soil aggregate-associated organic carbon (OC) and nutrients to tea cultivation age can lay a theoretical foundation for improving soil fertility, ensuring soil health, and promoting sustainable utilization of soil resources in the tea plantations. In this study, concentrations of soil OC and nutrient were analyzed in >2, 2-1, 1-0.25, and <0.25 mm fractions (split by a dry-sieving procedure) at the 0-20 cm soil layer in four tea plantations with Baimao tea of different ages (8, 17, 25, and 43 a) in southern Guangxi, China. The distribution of soil aggregates showed that the dominant aggregates were >2 mm fractions with a mean value of 63.8%, followed by <0.25 mm fractions, while 2-1 and 1-0.25 mm fractions with mean values of 9.9% and 7.8%, respectively. As an indicator of soil aggregate stability, the mean weight diameter (MWD) in the tea plantations showed an order of 17 a >8 a >25 a >43 a. Regardless of tea cultivation age, soil aggregate-associated OC and total nitrogen (TN) concentrations increased with increasing aggregate size. Soil OC and TN concentrations in >2 and 2-1 mm fractions were significantly higher than those in other fractions. The mean values of soil OC and TN concentrations were 18.76 and 0.84 g·kg-1 in the >2 mm fractions, and were 18.65 and 0.80 g·kg-1 in the 2-1 mm fraction. Soil aggregate-associated available nitrogen (AN), available phosphorus (AP), and availa-ble potassium (AK) concentrations were highest in the <0.25 mm fractions with mean values of 50.43, 23.06, and 68.04 mg·kg-1, respectively. Long-term tea cultivation was favorable to the accumulation of soil OC, TN, AN, and AP, whereas the accumulation rates of these element stocks in the whole soil decreased with increasing tea cultivation age. In contrast, soil AK was susceptible to leaching in tea cultivation, with the loss rate of this element stock in the middle stage (from 17 to 25 a) being higher than those in the other stages. To ensure soil quality and promote the sustainable utilization of soil resources, more attention should be paid to the problems such as the decrease of soil aggregate stability and the aggravation of AK loss after 17 a of tea cultivation.

Effects of Straw Return Mode on Soil Aggregates and Associated Carbon in the North China Plain

Crop straw is widely used to manage soil organic carbon (SOC) sequestration as an environmentally friendly practice in the North China Plain. However, little is known about the effects of straw returning modes on SOC sequestration in this region. Thus, a field experiment was conducted to study SOC accumulation and mineralization as well as aggregate stability and aggregate-associated SOC for the following three straw returning modes: no straw returning (NSR), only wheat straw returning (WR), and both wheat and maize straw returning (WR-MR). SOC concentration and storage were higher for WR and WR-MR than for NSR in the 0–20 cm soil layer, respectively (p &lt; 0.05). Although WR and WR-MR resulted in higher mineralization per unit of soil than NSR, no significant difference in mineralization per unit of soil carbon was observed among straw returning modes in the upper soil layer. The mean weight diameters of aggregates at 0–5 cm were higher under treatments with crop retention than under NSR. At this soil depth, the aggregate-associated C concentration and stock for each soil size were significantly decreased by NSR. These findings suggest that WR-MR and WR are effective residue management practices. In particular, WR is the optimal strategy to enhance SOC sequestration, considering other applications of straw (e.g., forage, fuel, or biomass).

Carbon sequestration and mineralization in soil aggregates under long-term conservation tillage in the North China Plain

Understanding the process of soil organic carbon (SOC) sequestration and mineralization in aggregates is pertinent to mitigate climate change and minimize risks of soil degradation. Thus, soil samples were obtained after a 10-year field experiment to identify the influences of tillage on aggregate-associated SOC sequestration and mineralization in the North China Plain (NCP). Four tillage practices investigated were as follows: no-till with straw retention (NTS, conservation tillage), rotary tillage with straw incorporation (RTS), conventional tillage with straw incorporation (CTS), and conventional tillage with straw removal (CT). Significantly negative correlations were observed between SOC concentration and potentially mineralized carbon in aggregates under different treatments for the 0–10 cm soil layer. The large macro-aggregates (>2 mm) with the highest proportion of size distribution represented the major pool of SOC stock (47.3–51.2%) and mineralization amount (38.2–43.6%) in the 0–30 cm layer, followed by that in the small macro-aggregates (0.25–2 mm), regardless of tillage practices. However, the mineralization quotient (mineralization per unit SOC concentration) of macro-aggregates (>0.25 mm) was lower than that for the other size classes. The NTS enhanced the macro-aggregate formation in the 0–20 cm layer and associated SOC concentration in the 0–10 cm layer. Furthermore, NTS decreased total potential mineralization in the 0–30 cm layer compared with the other tillage practices, attributed to decrease in the large macro-aggregates (30.0–51.4%) with low particulate organic carbon (POC) concentration. The NTS with low straw inputs had higher incremental efficiency with straw incorporation than that in the RTS and CTS by 45.0% and 13.5%, respectively (P < 0.05). Overall, the higher proportion of macro-aggregates recorded under NTS decreased carbon mineralization, and consequently, increased incremental efficiency with straw incorporation, and improved SOC sequestration in the surface soil layer in the NCP.

Soil organic carbon and aggregation in response to thirty-nine years of tillage management in the southeastern US

Agricultural management practices control soil organic carbon (SOC) content in croplands. Long-term cropping system experiments offer a great opportunity to understand the magnitude and direction of SOC change in response to management practices. Such information is very limited from the southeastern US, a region with warm and humid climatic conditions that typically favor SOC decomposition over accumulation. Therefore, this study was conducted to assess the effect of 39 years of chisel plow (CP), disc plow (DP), moldboard plow (MP), no-tillage (NT), NT with winter wheat (Triticum aestivum L.) cover crop (NTW), and NT with wheat-soybean (Glycine max L.) double crop (NTWD) on total SOC and SOC fractions including permanganate oxidizable C (POXC), water extractable C (WEC), resistant C (RC), and aggregate-associated SOC in a continuous soybean system. Additionally, aggregate size distribution, mean weight diameter (MWD), and wet aggregate stability (WAS) were determined. Results showed that NTW and NTWD significantly increased SOC and POXC compared to MP with mean SOC (g kg−1 soil) of 12.2 (NTW) ≥10.9 (NTWD) >7.2 (MP) and mean POXC (mg kg−1 soil) of 465 (NTWD) ≥418 (NTW) >252 (MP). The WEC and RC fractions did not differ among treatments. Across the treatments, the greatest aggregate-associated SOC concentration was found in microaggregates (0.053–0.25 mm) and the lowest in clay- and silt-size particles (<0.053 mm). Additionally, WAS under NT systems was significantly higher (45.5–52.3 %) than under tilled treatments (21.9–29.1 %). Total SOC correlated significantly with POXC (r = 0.68, p < 0.01), RC (r = 0.46, p < 0.05), WAS (r = 0.65, p < 0.01), and aggregate-associated SOC concentrations (r > 0.6, p < 0.01). Overall, this study revealed that NT enhanced SOC and POXC accumulation and macroaggregation compared to tilled treatments. Cover cropping and double cropping further improved SOC accumulation. In conclusion, long-term adoption of different tillage intensities can strongly alter SOC dynamics in bulk soil and aggregates under continuous soybean production systems of the southeastern US.

Increased soil aggregate stability is strongly correlated with root and soil properties along a gradient of secondary succession on the Loess Plateau

The stability of soil aggregates is an indicator of restoration of soil in degraded ecosystems. A multitude of factors such as properties of plant roots and soil have been suggested to contribute to aggregate stability, but little information is available on the relative importance of these factors in temperate grass zones. We examined how root and soil properties modified aggregate stability along a gradient of secondary succession grassland on the Loess Plateau in China. We selected three cropland abandoned for 3, 10, and 16-year and measured the distribution of aggregates, mean weight diameter (MWD), bulk and aggregate-associated soil organic carbon (SOC) and glomalin-related soil protein (GRSP) contents, root biomass density, root length density, and specific root length (SRL). Compared with 3-year site, the amount of large macroaggregates (>2 mm) and aggregate stability (indicated by MWD) at 16-year site increased by 25.6% and 8.5%. The higher MWD contributed the most to the accumulation of SOC in large and small macroaggregates and to the accumulation of GRSPs in microaggregates (<0.25 mm). SRL was significantly positively correlated with MWD. Redundancy analysis (RDA) showed that soil and plant variables together explained 89.1% of the aggregate distribution variation. Partial RDA further revealed that soil variables solely explained 6.4% of the variation, plant root variables explained 47.9% of the variation, and interaction of soil and plant variables accounted for 34.8% of the variation. Our study indicated that increased soil aggregate stability during plant secondary succession depended on both plant roots and aggregate-associated SOC and GRSPs, and plant root exerted a stronger influence on soil aggregate stability than soil. Allowing secondary succession may be a promising strategy for restoring degraded ecosystems on the plateau.

Rotation and Organic Fertilizers Stabilize Soil Water-Stable Aggregates and Their Associated Carbon and Nitrogen in Flue-Cured Tobacco Production

Long-term mono-cropping and excessive chemical fertilizer application reduce water-stable soil aggregate stability and soil N and C stocks in tobacco production. This study tested the hypothesis that rotation and fertilizer type affected the proportion of water-stable aggregates and aggregate-associated soil organic carbon (SOC) and total soil nitrogen (TSN) concentrations. Two planting systems (tobacco mono-cropping and tobacco-maize rotation) with five fertilizer treatments (0 and 75 kg N ha−1, 450 kg oil cake + 75 kg N ha−1, 15,000 kg farmyard manure + 60 kg N ha−1, and 3000 kg straw + 75 kg N ha−1) were established in 2007. After 10 years, rotation and fertilizer type significantly affected soil aggregates and associated SOC and TSN at different soil depths. Rotation significantly increased mean weight diameter (MWD) and geometric mean diameter (GMD); GMD increased with increasing soil depth. Rotation and organic fertilizer effectively increased the proportion of large macro-aggregates and MWD and GMD, while significantly reducing the proportion of micro-aggregates. The effect of fertilization type and rotation on the SOC content of large macro-aggregates was more obvious than that on TSN content, but there was no significant effect on SOC and TSN contents of aggregates 250 μm). However, this trend weakened with increasing soil depth and decreasing aggregate size. Rotation and organic fertilizers used in tobacco production stabilized the proportion of macro-aggregates in surface soil and maintained SOC and TSN better.

Tillage-induced effects on SOC through changes in aggregate stability and soil pore structure

Soil structure plays a key role in soil organic carbon (SOC) dynamics. To determine how soil structure and aggregate affects SOC, we collected undisturbed soil cores of 0–5 cm layer (Typic Hapludoll) at an experimental site in Northeast China. The site had been under continuous tillage treatments of conventional tillage (CT) and no tillage (NT) for 17 years. We measured SOC by elemental analysis, aggregate size distribution by wet sieving, and soil pore parameters of pore size distribution, pore average diameter, pore numbers, pore connectivity, pore anisotropy, and pore fractal dimension by X-ray computer tomography. SOC content was significantly correlated with aggregate-associated SOC and soil water-stable aggregate content. CT with residue removal and annual plowing and cultivation increased <53 μm and 53–250 μm aggregates. CT decreased total SOC of 0–5 cm soil layer but increased aggregate-associated SOC of <53 μm. NT with greater residue input increased total SOC of 0–5 cm soil layer by 26.0% and aggregate mean weight diameter by 111.8% and increased aggregates of 250–1000 μm and >1000 μm. Soil under NT had a greater total number of micropores and greater connectivity whereas CT had a greater total number of macropores, average macropore diameter, anisotropy, and fractal dimension. Structural equation modeling showed that CT can decrease SOC of 0–5 cm soil layer by different paths, including increased anisotropy and macropore porosity, and NT can increase SOC of 0–5 cm soil layer by different paths, including increased mean weight diameter and connectivity. These results enhance our understanding of the relationship between soil structure and SOC, and could guide tillage management decisions to increase SOC.

Response of oxidative stability of aggregate-associated soil organic carbon and deep soil carbon sequestration to zero-tillage in subtropical India

Conservation agriculture (CA) practices have been recognized to enhance soil health by optimizing key soil attributes and are being implemented worldwide. But, the potential of CA to soil organic carbon (SOC) sequestration and enhancing its oxidative stability (OXS) in deep layers and aggregates are less highlighted. Thus, our aim was to study the long-term (9 years) effects of contrasting tillage treatments and residue addition on soil aggregation, OXS in bulk soils (0–5, 5–15, 15–30 and 30–60 cm) and aggregates (0–5 and 5–15 cm), and C pools and sequestration (0–5, 5–15, 15–30 and 30–60 cm). For this an experiment was laid out in a split plot design with two tillage practices in main plots (zero-tillage: ZT and conventional tillage: CT) and four residue management practices in sub-plots (No residue: NR, wheat residue: WR, soybean residue: SR and wheat + soybean residue: WR + SR) in 2008. Soil samples collected from 0–5, 5–15, 15–30 and 30–60 cm layers were analyzed. The ZT plots had ∼26 and 15% higher macroaggregate associated C than CT plots in 0–5 and 5–15 cm soil layers, respectively. In macroaggregates of first two soil layers, recalcitrant C was (a) ∼57, 76 and 63%; and (b) 85, 59 and 40% greater in WR + SR, SR and WR plots over NR, respectively. Similar trend was observed for microaggregate associated recalcitrant C. The OXS of C in ZT was ∼21% more than CT, but residue addition had no impact on OXS. In 15–30 and 30–60 cm soil layers, OXS in WR + SR plots were ∼28 and 31% greater than NR. Although deep soil layers (15–60 cm) accounted for ∼68% of sequestered C, tillage had no impact on deep soil C sequestration. About ∼85% of total SOC stock was accumulated in soil aggregates of topsoil. Contrarily, of total sequestered SOC in top soil, only ∼30% was sequestered by aggregates of that layer indicating the role of silt + clay fraction in SOC sequestration in tropical Inceptisol. Thus, SOC sequestration in tropical soil is very sluggish and would require long time to meet saturation deficit. Interestingly, crop yield was significantly influenced by active and labile C of bulk soils and aggregates, but not by recalcitrant C in ZT plots. Thus, ZT in combination with crop residue retention has great potential in enhancing deep soil C sequestration and C stability of soil aggregates to sustain soil health and crop production.

Long-term fertilisation impact on temperature sensitivity of aggregate associated soil organic carbon in a sub-tropical inceptisol

Understanding impact of long-term fertilisation on soil aggregation, carbon (C) stabilisation and temperature sensitivity of soil organic C (SOC) mineralisation are necessary to foresee C dynamics. The present study was conducted in an Inceptisol under wheat (Triticum aestivum) based cropping system, with the objective to quantify the effect of 44 years of integrated nutrient management on soil aggregation, enzymatic activity, and temperature sensitivity (Q10) of SOC. Treatments were: unfertilised control (UC), 100% recommended dose of nitrogen (N), N and phosphorus (NP), N, P and potassium (NPK), integrated application of NPK and farmyard manure (FYM) (NPKF) and 150% recommended dose of N, P and K (1.5NPK). The observed data were analysed using analysis of variance for randomized block design and Tukey’s HSD test (P < 0.05) was used for mean separation. In the 0–15 cm soil layer, plots under NPKF had ˜121 and 19% larger macroaggregate-associated C than UC and NPK plots, respectively. Irrespective of soil depth and temperature, higher cumulative C mineralisation was observed for NPKF (by ˜67-91%) and NPK (by ˜42-56%) treated plots than UC. Activation energies for SOC decomposition in 0–15 cm bulk soil were ˜205, and 258% higher in NPKF, and 1.5NPK, respectively, than UC plots. Mean Q10 of macroaggregate-C was higher than microaggregate-C. Aggregate-C of sub-surface soil was more temperature sensitive than surface soil, possibly due to higher recalcitrant/total SOC in the subsurface than surface layer. Thus, it reveals the need of better C management practices to augment the lability of C in subsurface soil layers. Plots under NPKF had higher β–glucosidase and fluorescein diacetate activities in bulk soils and aggregates over NPK in the both layers. NPKF application is recommended for improved soil aggregation, SOC protection within macroaggregates, enzyme activities in bulk soils and aggregates and capable for less proportional SOC decomposition than NPK or UC plots at higher temperature. Thus, soil management involving organic manure application and mineral fertilisers would enhance the activation energy of SOC and act as a barrier for SOC decomposition by facilitating soil aggregation and enhance SOC sequestration.

Effects of different tillage and straw retention practices on soil aggregates and carbon and nitrogen sequestration in soils of the northwestern China

Soil tillage and straw retention in dryland areas may affect the soil aggregates and the distribution of total organic carbon. The aims of this study were to establish how different tillage and straw retention practices affect the soil aggregates and soil organic carbon (SOC) and total nitrogen (TN) contents in the aggregate fractions based on a long-term (approximately 15 years) field experiment in the semi-arid western Loess Plateau, northwestern China. The experiment included four soil treatments, i.e., conventional tillage with straw removed (T), conventional tillage with straw incorporated (TS), no tillage with straw removed (NT) and no tillage with straw retention (NTS), which were arranged in a complete randomized block design. The wet-sieving method was used to separate four size fractions of aggregates, namely, large macroaggregates (LA, >2000 μm), small macroaggregates (SA, 250–2000 μm), microaggregates (MA, 53–250 μm), and silt and clay (SC, <53 μm). Compared to the conventional tillage practices (including T and TS treatments), the percentages of the macroaggregate fractions (LA and SA) under the conservation tillage practices (including NT and NTS treatments) were increased by 41.2%–56.6%, with the NTS treatment having the greatest effect. For soil layers of 0–5, 5–10 and 10–30 cm, values of the mean weight diameter (MWD) under the TS and NTS treatments were 10.68%, 13.83% and 17.65%, respectively. They were 18.45%, 19.15% and 14.12% higher than those under the T treatment, respectively. The maximum contents of the aggregate-associated SOC and TN were detected in the SA fraction, with the greatest effect being observed for the NTS treatment. The SOC and TN contents were significantly higher under the NTS and TS treatments than under the T treatment. Also, the increases in SOC and TN levels were much higher in the straw-retention plots than in the straw-removed plots. The macroaggregates (including LA and SA fractions) were the major pools for SOC and TN, regardless of tillage practices, storing 3.25–6.81 g C/kg soil and 0.34–0.62 g N/kg soil. Based on the above results, we recommend the NTS treatment as the best option to boost soil aggregates and to reinforce carbon and nitrogen sequestration in soils in the semi-arid western Loess Plateau of northwestern China.

Distribution of soil aggregates and organic carbon in deep soil under long-term conservation tillage with residual retention in dryland

To ascertain the effects of long-term conservation tillage and residue retention on soil organic carbon (SOC) content and aggregate distribution in a deep soil (>20-cm depth) in a dryland environment, this paper analyzed the SOC and aggregate distribution in soil, and the aggregate-associated organic carbon (OC) and SOC physical fractions. Conservation tillage (reduced tillage with residue incorporated (RT) and no-tillage with residue mulch (NT)) significantly increased SOC sequestration and soil aggregation in deep soil compared with conventional tillage with residue removal (CT). Compared with CT, RT significantly increased the proportion of small macroaggregates by 23%–81% in the 10–80 cm layer, and the OC content in small macroaggregates by 1%–58% in the 0–80 cm layer. RT significantly increased (by 24%–90%) the OC content in mineral-SOC within small macroaggregates in the 0–60 cm layer, while there was a 23%–80% increase in the 0–40 cm layer with NT. These results indicated that: (1) conservation tillage treatments are beneficial for soil aggregation and SOC sequestration in a deep soil in a dryland environment; and (2) the SOC in mineral-associated OC plays important roles in soil aggregation and SOC sequestration. In conclusion, RT with NT is recommended as an agricultural management tool in dryland soils because of its role in improving soil aggregation and SOC sequestration.

Soil organic carbon and nutrients associated with aggregate fractions in a chronosequence of tea plantations

Research on the changes of soil aggregate-associated organic carbon (C) and nutrients in different aged tea (Camellia sinensis L.) plantations is essential for improving our understanding of soil organic C mitigation and nutrient restitution during tea cultivation. However, the short- and long-term implications of the conversion of abandoned land to tea plantations on these elements associated with aggregate fractions remain poorly understood. In this study, soil organic C, total nitrogen (N), available phosphorus (P), as well as exchangeable cations, namely potassium (K+), sodium (Na+), calcium (Ca2+), and magnesium (Mg2+), were analyzed in different size fractions of soil aggregates that were collected from 0 to 20 cm depth in four tea plantations with the same cultivar (Sichuan tea) of various ages (16-, 23-, 31-, and 53-years) in the hilly area of western Sichuan in China. Soil aggregates were classified into six fractions by a wet-sieving procedure, namely >5, 5–2, 2–1, 1–0.5, 0.5–0.25, and <0.25 mm. As main carriers of soil organic C and nutrients, both >5 and <0.25 mm fractions had relatively high stocks of these elements in all the tea plantations. During the 37-years of tea cultivation (from 16- to 53-years), the accumulation rates of soil organic C, total N, and available P were 40.32 g C m−2 year−1, 1.43 g N m−2 year−1, and 0.03 g P m−2 year−1, respectively, while the loss rates of soil total exchangeable bases (TEB) were 3.54 cmol TEB m−2 year−1. Notably, the change rates of these element stocks in the whole-soil differed with the stage of tea cultivation. For example, using the stock of soil organic C in the 16-year-old tea plantation as a reference point, an average increase in soil organic C stock of 96.09 g C m−2 year−1 occurred in the early (from 16- to 23-years) stage, while the increase was 51.70 g C m−2 year−1 in the middle (from 23- to 31-years) and 18.43 g C m−2 year−1 in the later (from 31- to 53-years) stages. Similar trends were observed in the soil total N and available P. Furthermore, an average decrease in soil TEB stocks of 2.83 cmol TEB m−2 year−1 occurred in the early stage, while the decrease was 12.67 cmol TEB m−2 year−1 in the middle and 0.80 cmol TEB m−2 year−1 in the later stages. These results indicated that long-term tea cultivation brought various benefits, such as the accumulation of soil organic C, total N, and available P. However, their accumulation rates in the whole-soil decreased with the increase of tea plantation age. In addition, it is important to establish sustainable management practices to minimize soil exchangeable cations loss during the tea planting process, especially after 23-years, in the hilly area of western Sichuan in China.

Distribution of soil organic carbon within aggregate size fractions following the conversion of cropland to black locust forest on the southern Loess Plateau, China

The purpose of this study was to elucidate the effect of cropland conversion to black locust (Robinia pseudoacacia L.) forest on soil organic carbon (SOC) contents within aggregate size fractions on the slopes of the southern Loess Plateau, China. Soil samples were collected from six sites at 0 to 20 cm and 20 to 40 cm in duplicate. Five sites were artificial black locust forests that had been converted from cropland 6 years (FL06), 15 years (FL15), 29 years (FL29), 40 years (FL40), and 56 years (FL56) ago, and one neighboring site was cropland (CK) cultivated with wheat (Triticum aestivum L.). Five aggregate size fractions (>2 mm, 1 to 2 mm, 0.5 to 1 mm, 0.25 to 0.5 mm, and 2 mm and 2 mm and 2 mm and 1 to 2 mm aggregate fractions under the locust forests increased by 11% to 192% and 15% to 230%, respectively. The contribution ratio (R) of the aggregate-associated SOC to the corresponding total SOC in

Soil aggregate-associated organic carbon dynamics subjected to different types of land use: Evidence from 13C natural abundance

The technique of 13C natural abundance provides new information that can be applied to study carbon (C) incorporation into soil aggregates and to improve our understanding of the aggregate hierarchy theory. The organic matter (OM) in soil undergoes microbial decomposition, which preferentially removes the lighter 12C isotope, enriching the remaining soil in the heavier 13C isotope. We hypothesized that the soil aggregate turnover would gradually lead to C isotope fractionation, and changes in the 13C natural abundance are closely linked to C turnover that, in turn, is influenced by aggregates turnover. We examined how land use affects soil organic carbon (SOC) dynamics in soil aggregate with the following objectives: (1) evaluate the influence of land uses (woodland, orchard, paddy and upland) on aggregate-associated C isotope compositions and the distribution of new and old SOC in the Danjiangkou Reservoir area of central China, and (2) propose an extended scheme of C transfers between the aggregate size classes. The results showed that SOC content in aggregates generally increased with increasing aggregate size but the δ13C values of aggregates decreased. The SOC contents in bulk soil decreased in the order of paddy > woodland > upland > orchard. The most negative δ13C values in bulk soil and aggregates were observed in the paddy and woodland soils, whereas maximum δ13C values were obtained in the orchard and upland soils. The stable C isotope results suggest that SOC sequestration of fresh OM generally starts in macro-aggregates (>0.25 mm), and, after disaggregation processes and microbiological consumption, the resulting degraded OM is sequestered in micro-aggregates (<0.25 mm). This general trend is consistent with the concept of SOM stabilization by association with aggregate hierarchy theory. We conclude that different land uses and management practices significantly affect C incorporation in the aggregate system and C transfer in soil aggregates was considerably greater in the orchard and upland soils than in the woodland and paddy soils. These findings help improve the theory of soil aggregates.

Effects of freeze-thaw cycles on aggregate-associated organic carbon and glomalin-related soil protein in natural-succession grassland and Chinese pine forest on the Loess Plateau

Glomalin-related soil protein (GRSP) is a hydrophobic glycoprotein that can protect soil organic carbon (SOC) from decomposition by promoting the formation of soil aggregates. Few studies, however, have evaluated the response of aggregate-associated SOC and GRSP to freeze-thaw cycles (FTs) under different patterns of vegetation restoration. Soil samples were collected from three typical natural-succession grasslands (dominated by Artemisia capillaris, A. sacrorum, and Bothriochloa ischaemum) and a Chinese pine (Pinus tabuliformis) forest before and after natural FTs during a winter-spring season on the Loess Plateau in China. The soil aggregate stability and concentrations of SOC, easily extractable GRSP (EE-GRSP) and total GRSP (T-GRSP) in different aggregate fractions were measured. The FTs decreased the proportion of >5.0 mm wet-sieved fractions, increased the proportion of <0.25 mm wet-sieved fractions, and decreased the mean weight diameters by 8.7, 9.5, 2.9, and 9.4% in the soils of A. capillaris, A. sacrorum, B. ischaemum, and P. tabuliformis, respectively. The SOC, T-GRSP, and EE-GRSP concentrations in each aggregate fraction generally decreased after FTs in the four vegetation types. The SOC, T-GRSP, and EE-GRSP concentrations did not differ significantly in the larger aggregate fractions before the FTs in the soils for B. ischaemum and P. tabuliformis, but their concentrations were significantly higher for B. ischaemum than P. tabuliformis after FTs. The SOC and GRSP concentrations were significantly and linearly correlated in most aggregate fractions both before and after the FTs. These results indicated that the FTs decreased SOC by decreasing the GRSP concentration in each aggregate fraction and that SOC was more sensitive to FTs in forest than in natural-succession grassland.

Aggregate stability and associated organic carbon and nitrogen as affected by soil erosion and vegetation rehabilitation on the Loess Plateau

Although soil erosion and land use change have long been focuses in carbon research, the combined influence of soil erosion and vegetation rehabilitation on aggregate stability and the associated soil organic carbon (SOC) and total nitrogen (TN) remains unclear. The current study evaluated the effects of soil erosion on aggregate stability and the associated SOC and TN dynamics in relation to vegetation rehabilitation after the implementation of the “Grain-for-Green” project in the hilly Loess region. A check dam sediment sequence was dated using 137Cs activity and erosive rainfall events. The SOC and TN in the bulk soil and aggregate fractions were measured in soils from rehabilitated grasslands and sloping croplands and in sediments retained by the check dam. The results showed that vegetation rehabilitation led to 78%, 27% and 9% average increases in the macroaggregate amount, mean weight diameter (MWD) and mean geometric diameter (MGD), respectively. In addition, rehabilitation resulted in the highest SOC and TN concentrations and contents in macroaggregates among all the aggregate size fractions. Soil erosion facilitated the modification of the aggregate size distributions along with soil mineralization and induced the incorporation of deeper SOC-poor soils during transport. These processes resulted in the aggregate-associated SOC and TN concentrations and contents in the sediments being significantly lower than those in the eroding sloping cropland soils. The highest reductions were found in microaggregates, which exhibited decreases of 48% and 44% for SOC and TN, respectively. Moreover, reaggregation and gully soils incorporated during soil erosion led to higher values of macroaggregate amount and aggregate stability at depositional sites than those at eroding sloping cropland sites in this study. Our study contributes to the understanding of the effects of soil erosion and vegetation rehabilitation on SOC and TN dynamics, which is crucial for understanding the restoration efficiency in soil erosion control and ecosystem security evaluation.

Rotation and manure amendment increase soil macro-aggregates and associated carbon and nitrogen stocks in flue-cured tobacco production

Flue-cured tobacco production in China is typically over-fertilised and mono-cropped. To understand how this agronomic management affects soil structure and organic matter, this study investigated the effect of rotation, fertilizer rate, and manure amendment on the proportion of water stable aggregates and aggregate-associated soil organic carbon (SOC) and total soil nitrogen (TSN) concentrations and stocks in tobacco production. Two tobacco management systems (Tobacco monoculture and Tobacco-rice rotation) with four fertilizer treatments (0, 75, and 112 kg N/ha, and 60 kg N/ha + manure) were established in 1998. After 18 years, soil aggregation and aggregate-associated SOC and TSN were significantly affected by rotation and fertilizer management. Compared to tobacco monoculture and current fertilizer management, rotation and manure amendment increased macroaggregate (>250 μm) proportion and geometric mean diameter and decreased the proportion of microaggregates and silt-clay sized fractions (<250 μm). Simultaneously, rotation and manure amendment increased macroaggregate-associated SOC and TSN stocks at the expense of the microaggregate and silt-clay size class and their associated SOC and TSN stocks. Rotation and/or manure treatment can maintain satisfactory physico-chemical soil properties through macroaggregate stabilization in tobacco production, which contributes to conserving SOC and TSN stocks.