Intra-aggregate Particulate Organic Matter Research Articles

Natural revegetation over ∼ 160 years alters carbon and nitrogen sequestration and stabilization in soil organic matter on the Loess Plateau of China

Natural revegetation has been reported to play a very active role in ecosystem carbon (C) and nitrogen (N) sinks in degenerated ecosystems. However, the responses of C and N sequestration and stabilization in soil organic matter (SOM) to natural revegetation remain inadequately understood. In this study, we analyzed C and N contents and δ13C and δ15N values of SOM, free light fraction (FLF), intra-aggregate particulate organic matter (IPOM) and mineral-associated organic matter (MAOM) along ∼ 160 years of natural revegetation on the Loess Plateau of China. The results showed that natural revegetation significantly (P < 0.05) enhanced soil C and N contents in FLF, IPOM, MAOM, SOM in the surface soil (0–20 cm) during the later stages of revegetation, which exhibited smaller impact on the deeper soil (20–60 cm). Natural revegetation significantly increased distribution proportions of C and N in FLF (i.e., 18.00–36.00%, 8.46–22.57%, respectively), and that in IPOM (i.e., from 11.60 to 25.38%, 10.89–26.92%, respectively), while it decreased that in MAOM (i.e., from 56.25 to 37.00%, 67.69–50.24%, respectively) in the surface soil, thereby altering C and N stabilization in SOM (0–20 cm). The climax Quercus liaotungensis forest exhibited the highest C distribution proportion in FLF and IPOM, and the lowest C distribution proportion in MAOM in the surface soil. The δ13C and δ15N were enriched with the decomposition of SOM and soil profile depths at each revegetation stage. The highest δ13C and δ15N values (0–60 cm) and the lowest C:N ratio (0–20 cm) of SOM, FLF, and IPOM were found in the farmland. In conclusion, ∼160 years of natural revegetation substantially promoted C and N sequestration in SOM, whereas altered C and N stabilization in SOM of the surface soil through shifting C and N towards more non-protected and pure physically protected SOM fractions rather than the most stable MAOM. Meanwhile, soil δ13C and δ15N in SOM and its fraction were changed along with natural revegetation. The most enriched soil δ15N in the farmland implied that soil N cycle in the farmland was more open to N losses relative to the other revegetation stages.

60 years of fertilization and liming impacts on soil organic carbon stabilization in a sub-tropical Alfisol.

Limited information is available on the C stabilization mechanism of tropical soils under different management practices including long-term organic manuring, mineral fertilization alone, or in combination with lime. Hence, to understand the effect of continuous application (for 60 years) of organic manure, fertilizer, and lime alone or in combination on an acidic Alfisol, stabilization of soil organic carbon (SOC) was evaluated under maize (Zea mays L.) wheat (Triticum aestivum L.) cropping. There were eight treatments that included farmyard manure (FYM) and nitrogen (N) applied in terms of FYM, additional dose of phosphorus (P) and potassium (K) applied in terms of inorganic fertilizer (FYM + P'K'), FYM + P'K' with liming (FYM + P'K' + L) and NPK alone. These treatments were laid in a randomized block design with three replications. Results indicated that FYM + P'K' plots had maximum amount of SOC inside large macroaggregates. The value was 33 and 92% greater than only minerally fertilized (NPK) and unfertilized control plots, respectively, whereas microaggregate-associated C was highest in plots with FYM + P'K' and lime (FYM + P'K' + L), which was 48 and 183% more than unfertilized control and NPK plots, respectively. Inside soil microaggregates, plots under FYM + P'K' had highest labile C, while NPK + L plots had highest recalcitrant C. Plots with organic amendments contained higher glomalin in large macroaggregates. Plots treated with FYM + P'K' had maximum intra-aggregate particulate organic matter within microaggregates inside macroaggregates (iPOM_mM), which was 28 and 74% higher than NPK and unfertilized control plots, respectively. Total C stock inside the protected microaggregates within macroaggregates was maximum for FYM + P'K' plots. It had 38, 67, and 171% higher C stock than NPK, FYM, and unfertilized control plots, respectively. Interestingly, despite estimated C input in FYM-treated plots was much higher than NPK plots, FYM-treated plots had less C stabilization within microaggregates and within microaggregates inside macroaggregates. Microaggregates within macroaggregates accounted for ~54% of the recalcitrant C content. Thus, macroaggregates stabilization through occlusion of microaggregates was accountable for sequestration of SOC and only FYM application did not promote that mechanism compared to NPK. Carbon stabilization within macroaggregates under FYM plots was mainly governed by amorphous iron oxide.

Microbial metabolic response to winter warming stabilizes soil carbon.

Current consensus on global climate change predicts warming trends with more pronounced temperature changes in winter than summer in the Northern Hemisphere at high latitudes. Moderate increases in soil temperature are generally related to faster rates of soil organic carbon (SOC) decomposition in Northern ecosystems, but there is evidence that SOC stocks have remained remarkably stable or even increased on the Tibetan Plateau under these conditions. This intriguing observation points to altered soil microbial mediation of carbon-cycling feedbacks in this region that might be related to seasonal warming. This study investigated the unexplained SOC stabilization observed on the Tibetan Plateau by quantifying microbial responses to experimental seasonal warming in a typical alpine meadow. Ecosystem respiration was reduced by 17%-38% under winter warming compared with year-round warming or no warming and coincided with decreased abundances of fungi and functional genes that control labile and stable organic carbon decomposition. Compared with year-round warming, winter warming slowed macroaggregate turnover rates by 1.6 times, increased fine intra-aggregate particulate organic matter content by 75%, and increased carbon stabilized in microaggregates within stable macroaggregates by 56%. Larger bacterial "necromass" (amino sugars) concentrations in soil under winter warming coincided with a 12% increase in carboxyl-C. These results indicate the enhanced physical preservation of SOC under winter warming and emphasize the role of soil microorganisms in aggregate life cycles. In summary, the divergent responses of SOC persistence in soils exposed to winter warming compared to year-round warming are explained by the slowing of microbial decomposition but increasing physical protection of microbially derived organic compounds. Consequently, the soil microbial response to winter warming on the Tibetan Plateau may cause negative feedbacks to global climate change and should be considered in Earth system models.

Zero tillage and residue retention impact on soil aggregation and carbon stabilization within aggregates in subtropical India

Studying the impact of different tillage systems on soil organic matter (SOM) dynamics is essential to define better strategies to improve soil fertility and soil organic carbon (SOC) sequestration. We used density fractionation to assess the changes in SOM in an Inceptisol following nine-years of contrasting tillage under wheat (Triticum aestivum) based cropping systems. The major objective of the study was to investigate the amount of C content within soil aggregates as influenced by tillage and residue management. For this a nine-year old experiment was conducted 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). The results indicated that SOC content increased by ∼48, 42 and 36%, respectively, in WR + SR, SR and WR plots compared with the NR plots in the 0-5 cm soil layer. The SOC content within macroaggregates were ∼30 and 25% higher in the ZT plots than CT in the 0-5 and 5-15 cm soil depths, respectively. The intra-aggregate particulate organic matter (iPOM) inside microaggregates within macroaggregates (iPOM_mM) was ∼16% higher in the CT than the ZT plots. Tillage management had no significant effect on light fraction inside microaggregates within macroaggregates (LF_mM) and iPOM_mM SOC contents inside microaggregates within macroaggregates (mM) in the surface soil, but residue management improved LF_mM and iPOM_mM SOC contents. The WR + SR, SR and WR plots had ∼73, 40 and 35% more LF_mM SOC content, respectively, than NR plots. Within free microaggregates in the surface soil, the LF_m SOC content was significantly enhanced due to ZT operation by ∼61% than the CT plots but not by crop residue addition. The iPOM_mM and iPOM_m fraction accounted for about ∼35 and 51% of the total SOC content within macroaggregates and microaggregates, respectively. That indicates the role of iPOM fraction in SOC sequestration within the soil aggregates in tropical Inceptisol. Plots under ZT had ∼38% more C content inside microaggregates within macroaggregates (mM) than CT (2.31 Mg ha-1), indicating ZT management could be adopted for C stabilization within aggregates.

Feedbacks between nitrogen fixation and soil organic matter increase ecosystem functions in diversified agroecosystems.

Nitrogen (N) losses from intensified agriculture are a major cause of global change, due to nitrate (NO3- ) export and the eutrophication of aquatic systems as well as emissions of nitrous oxide (N2 O) into the atmosphere. Diversified agroecosystems with legume cover crops couple N and carbon (C) inputs to soil and reduce N pollution, but there is a need to identify controls on legume N2 fixation across ecosystems with variable soil conditions. Here, I tested the hypothesis that N mineralization from turnover of soil organic matter (SOM) regulates legume N2 fixation across 10 farms that spanned a gradient of SOM levels. I separated soil samples into two SOM fractions, based on size and density, which are indicators of soil nutrient cycling and N availability (free particulate organic matter and intra-aggregate particulate organic matter [POM]). This study indicates downregulation of legume N2 fixation in diversified agroecosystems with increasing N availability in intra-aggregate POM and increasing N mineralization. Intercropping the legume with a grass weakened the relationship between N in POM and N2 fixation due to N assimilation by the grass. Further, mean rates of N and C mineralization across sites increased with two seasons of a legume-grass cover crop mixture, which could enhance this stabilizing feedback between soil N availability and N2 fixation over time. These results suggest a potential mechanism for the diversity-ecosystem-function relationships measured in long-term studies of agroecosystems, in which regular use of legume cover crops increases total soil organic C and N and reduces negative environmental impacts of crop production.

Soil aggregate and intra-aggregate carbon fractions associated with vegetation succession in an alpine wetland of Northwest China

Alpine wetlands can function as carbon sinks because of their high soil organic content and low decomposition rates. However, the effects of alpine wetland succession on soil structure and soil organic carbon (SOC) stability have rarely been reported. The objective of this study was to investigate the effects of vegetation succession on soil aggregate distribution, bulk soil carbon, aggregate-associated C, and intra-aggregate C fraction in the Bayinbuluk alpine wetland. Research sites were selected in adjacent swamp (S), swampy meadow (SM), and meadow (M) locations. Five soil aggregate fractions (>5000, 2000–5000, 250–2000, 53–250 and <53 μm) were separated using a modified Yoder method. The light fraction organic carbon (LFOC), coarse/fine intra-aggregate particulate organic matter carbon (iPOM-C), and mineral-associated organic matter carbon (mSOM-C) were isolated from soil aggregates. In the wetland succession sequence, the mean weight diameter of soil aggregates increased (from 4.16 to 4.63 mm) from S to SM, and then decreased (from 4.63 to 3.23 mm) from SM to M. The bulk soil carbon content initially increased (from 93 to 129 mg kg−1) from S to SM, and then decreased (from 129 to 54 mg kg−1) from SM to M. The same trends were found for the aggregate-associated C in different size aggregates. The coarse iPOM-C content (88–133 mg kg−1) was higher than the fine iPOM-C content (77–118 mg kg−1) in different size aggregates in S samples. In SM and M samples, however, the coarse iPOM-C contents (98–124 mg kg−1 (SM); 60–82 mg kg−1 (M)) were lower than the fine iPOM-C contents (129–151 mg kg−1 (SM); 71–103 mg kg−1 (M)). The ratio of coarse iPOM-C to fine iPOM-C content (ROC/F) exceeded 1 in S but was lower than 1 in SM and M. These results indicate that soil aggregation stability and SOC content increased and then decreased across the wetland succession sequence, and that ROC/F can be used as an indicator of changes in SOC content caused by wetland succession.

Fluvial sedimentary deposits as carbon sinks: organic carbon pools and stabilization mechanisms across a Mediterranean catchment

Abstract. The role of fluvial sedimentary areas as organic carbon sinks remains largely unquantified. Little is known about mechanisms of organic carbon (OC) stabilization in alluvial sediments in semiarid and subhumid catchments where those mechanisms are quite complex because sediments are often redistributed and exposed to a range of environmental conditions in intermittent and perennial fluvial courses within the same catchment. The main goal of this study was to evaluate the contribution of transport and depositional areas as sources or sinks of CO2 at the catchment scale. We used physical and chemical organic matter fractionation techniques and basal respiration rates in samples representative of the three phases of the erosion process within the catchment: (i) detachment, representing the main sediment sources from forests and agricultural upland soils, as well as fluvial lateral banks; (ii) transport, representing suspended load and bedload in the main channel; and (iii) depositional areas along the channel, downstream in alluvial wedges, and in the reservoir at the outlet of the catchment, representative of medium- and long-term residence deposits, respectively. Our results show that most of the sediments transported and deposited downstream come from agricultural upland soils and fluvial lateral bank sources, where the physicochemical protection of OC is much lower than that of the forest soils, which are less sensitive to erosion. The protection of OC in forest soils and alluvial wedges (medium-term depositional areas) was mainly driven by physical protection (OC within aggregates), while chemical protection of OC (OC adhesion to soil mineral particles) was observed in the fluvial lateral banks. However, in the remaining sediment sources, in sediments during transport, and after deposition in the reservoir (long-term deposit), both mechanisms are equally relevant. Mineralization of the most labile OC (the intra-aggregate particulate organic matter (Mpom) was predominant during transport. Aggregate formation and OC accumulation, mainly associated with macroaggregates and occluded microaggregates within macroaggregates, were predominant in the upper layer of depositional areas. However, OC was highly protected and stabilized at the deeper layers, mainly in the long-term deposits (reservoir), being even more protected than the OC from the most eroding sources (agricultural soils and fluvial lateral banks). Altogether our results show that both medium- and long-term depositional areas can play an important role in erosive areas within catchments, compensating for OC losses from the eroded sources and functioning as C sinks.

Effect of straw return mode on soil aggregation and aggregate carbon content in an annual maize-wheat double cropping system

Crop residue is a commonly used organic soil amendment in summer maize (June–October)–winter wheat (October–June of next year) rotation systems. However, the effects of different straw return modes on soil aggregation and soil organic carbon (SOC) stocks in different water-stable aggregates have not been extensively investigated in these cropping systems. The objective of this study was to quantify the long-term (7 yr) impact of C input on the SOC content of four soil aggregate size classes (large macroaggregates; small macroaggregates; microaggregates, silt plus clay fraction) and in explicit SOC fractions (free light fraction, free LF; intra-aggregate particulate organic matter, iPOM; mineral-associated matter, mSOM) within the top 40cm of soil in a wheat–maize double cropping system in Northwest China. Four treatments were examined: (i) no return (control); (ii) return of wheat straw only (WR); (iii) return of maize straw only (MR); and (iV) return of both maize and wheat straw (MR-WR). Over the experimental period, the change in SOC under the four treatments ranged from −0.96 to 5.83Mgha−1 and a significant linear relationship between SOC change and cumulative C input (R2=0.9882, P<0.05) was observed. Relative to the control, the proportion of large and small macroaggregates in the 0–20cm soil layer increased the most in MR-WR (32% and 24%), followed by MR (22% and 13%), and WR (11% and 10%). Straw return significantly increased the SOC content in each soil aggregate size class relative to no straw return. The order of SOC fractions with respect to SOC content was mSOM>fine iPOM>coarse iPOM>free LF. Straw return significantly increased the C stock in iPOM and mSOM relative to the control. Coarse iPOM was the most sensitive indicator of C change and mSOM was the main form of SOC under long-term straw return. A significant linear relationship existed between cumulative C input and the mass proportion of macroaggregates as well as the C content of SOC fractions (or aggregate fractions). Soil depth had a significant influence on almost all measurements, with greater values observed in the 0–20cm layer than in the 20–40cm layer. Overall, return of both maize and wheat straw was the best strategy for improving soil structure, soil organic carbon, and crop yield. However, straw return from one crop was sufficient to maintain initial SOC levels, and conserved straw could be used for cellulosic feedstocks.

Dynamics of physically- separated soil organic carbon pools assessed from δ13C changes under 25 years of cropping systems

Quantification of the dynamics of soil organic carbon (SOC) pools under the influence of different cropping systems is essential for predicting carbon (C) sequestration. We combined soil fractionation with C isotope analyses to investigate the SOC dynamics of the various soil fractions in a black soil of Northeast China after 25 years of tillage. Soil samples from the initial condition (CK) and 2 cropping treatments including continuous maize cropping (MM), maize–soybean rotation (MS) were separated into 4 aggregate sizes (<53μm, 250–53μm, 2000–250μm, and >2000μm) and 3 density fractions: free light fraction (LF), intra-aggregate particulate organic matter (iPOM), and mineral-associated organic matter (mSOM). The 25 years of cropping with manure application significantly increased the SOC storage, mainly by enhancing the soil C of the macroaggregates (2000–250μm), with most of the C stored in the iPOM (62.01–90.32%). The MS system was more beneficial for the SOC accumulation in macroaggregates (>250μm) than the MM system because of enhanced SOC in heavier fractions (iPOM and mSOM); this was probably induced by the differentiation of the belowground humification rate between soybean and maize roots, while the MM system may be a more effective measure for future soil C sequestration because most of the stable C is stored in the small size fraction (<53μm). The δ13C values indicated that, among aggregate sizes, the fastest soil C turnover occurred in microaggregates (250–53μm). Moreover, C in the MS soils had a faster turnover rate than in the MM soils.

Soil organic carbon and nitrogen dynamics following Spartina alterniflora invasion in a coastal wetland of eastern China

Plant invasion has been reported to modify ecosystem carbon (C) and nitrogen (N) cycling processes and pools. The aims of this work were to identify the consequences of plant invasion on C and N dynamics in soil organic matter (SOM) and its fractions following invasion chronosequence. In this study, we investigated the soil C and N contents and the δ13C and δ15N values of the SOM, free light fraction (LF), intra-aggregate particulate organic matter (iPOM) and mineral-associated organic matter (mSOM) in invasive 6-, 10-, 17- and 20-year-old Spartina alterniflora communities and compared with bare flat land in a coastal wetland of eastern China. S. alterniflora invasion significantly increased soil organic carbon (SOC) and soil organic nitrogen (SON) contents in SOM fractions compared with the bare flat land. The SOC, iPOM-C and iPOM-N progressively increased, but the SON, mSOM-C and mSOM-N did not significantly change following S. alterniflora invasion chronosequence. The proportion of C in iPOM gradually increased, whereas that in mSOM decreased following S. alterniflora invasion chronosequence except for the 20-year-old S. alterniflora soil. The highest proportion of S. alterniflora-derived C in SOM and its fractions was found in 17-year-old soil covered by S. alterniflora. The decay rate of old C decreased and the mean residence time of C increased in SOM and its fractions following S. alterniflora invasion chronosequence. The δ15N values of soil under S. alterniflora were more enriched than were those in bare flat land, whereas the δ15N values depleted following invasion chronosequence. Our results show that S. alterniflora invasion significantly altered SOC dynamics following invasion chronosequence by changing C physical distribution in SOM, S. alterniflora-derived C input and C decomposition, ultimately having a greater impact on SOC incorporation relative to SON accumulation in a coastal wetland of eastern China.

Variation of soil aggregation and intra-aggregate carbon by long-term fertilization with aggregate formation in a grey desert soil

A long-term field experiment was established in 1990 to evaluate the impact of mineral fertilizer (N and NPK) and organic material (manure and crop residues) application on soil aggregation and organic carbon (C) accumulation in an intensively cultivated grey desert soil in northwestern China. The long-term addition of organic materials with balanced inorganic NPK inputs significantly increased the mass proportion of macroaggregates from 10% in the CK treatment to 34% and 24% in the 1.5MNPK and SNPK treatments (the application of inorganic NPK combined with organic sheep manure and the return of crop residues to the field, respectively). In contrast, the addition of inorganic fertilizer (the N and NPK treatments) had no obvious effect in promoting the percentage of macroaggregates. The application of inorganic fertilizer and abandonment (the CK0 treatment) only sustained the soil organic carbon (SOC) content. However, the SOC contents were significantly increased by 197% and 19% in the 1.5MNPK and SNPK treatments, respectively, relative to the CK treatment over a 24-year period. Amending the soils with N and NPK mainly increased the organic C concentrations in the silt+clay fraction. In contrast, the application of manure accelerated the accumulation of organic C in all of the aggregate fractions, particularly by increasing the amount of C in the fine intra-aggregate particulate organic matter (iPOM) fraction rather than the coarse iPOM or the silt+clay subfraction. The organic C concentration in the silt+clay fraction was significantly correlated with the mass proportion of macroaggregates (R2=0.9488, P<0.01), and with the ratio of macroaggregates to microaggregates (R2=0.9477, P<0.01). These results indicated that macroaggregates and the silt+clay fraction were more sensitive to fertilization in the grey desert soils. We consider that increases in the organic C concentrations in the silt+clay fraction and shifts in the stored C towards microaggregates may play important roles in soil aggregation and SOC sequestration in grey desert soils.

Soil organic carbon dynamics under long-term fertilization in a black soil of China: Evidence from stable C isotopes.

Effects of different fertilizers on organic carbon (C) storage and turnover of soil fractions remains unclear. We combined soil fractionation with isotope analyses to examine soil organic carbon (SOC) dynamics after 25 years of fertilization. Five types of soil samples including the initial level (CK) and four fertilization treatments (inorganic nitrogen fertilizer, N; balanced inorganic fertilizer, NPK; inorganic fertilizer plus farmyard manure, MNPK; inorganic fertilizer plus corn straw residue, SNPK) were separated into four aggregate sizes (>2000 μm, 2000–250 μm, 250–53 μm, and <53 μm), and three density fractions: free light fraction (LF), intra-aggregate particulate organic matter (iPOM), and mineral-associated organic matter (mSOM). Physical fractionation showed the iPOM fraction of aggregates dominated C storage, averaging 76.87% of SOC storage. Overall, application of N and NPK fertilizers cannot significantly increase the SOC storage but enhanced C in mSOM of aggregates, whereas MNPK fertilizer resulted in the greatest amount of SOC storage (about 5221.5 g C m2) because of the enhanced SOC in LF, iPOM and mSOM of each aggregate. The SNPK fertilizer increased SOC storage in >250 μm aggregates but reduced SOC storage in <250 μm aggregates due to SOC changes in LF and iPOM.

Shifts in soil organic carbon and nitrogen dynamics for afforestation in central China

The afforestation has been proposed as a primary means of sequestering carbon (C) from the atmosphere, thereby mitigating climate change. However, consequences of afforestation on soil organic C and nitrogen (N) dynamics due to spatial heterogeneity are not fully understood. The objectives of this study were to identify the consequences of 18 years of afforestation on C and N dynamics in the Danjiangkou Reservoir area of central China. Soil samples from the woodland, shrubland, cropland and adjacent open area soils (i.e. the control) were separated into four aggregate sizes (>2000μm, 250–2000μm, 53–250μm and <53μm), and three density fractions [free light fraction (LF), intra-aggregate particulate organic matter (iPOM) and mineral-associated organic matter (mSOM)]. All fractions were analyzed for their C and N content, and δ13C and δ15N values. Afforestation enhanced the soil C and N storage, primarily due to increases in soil C and N storage of macroaggregates (>2000μm) with the largest (65–87%) fraction in iPOM. The C: N ratios generally increased from the cropland to shrubland to woodland across all fractions. The 13C values indicated that the fastest decay rates (k=0.024yr−1) in LF of macroaggregates (>2000μm) was in woodland, but the fastest decay rates (0.114–0.137yr−1) in iPOM of all aggregates were observed in cropland. Meanwhile, the most enriched δ15N values (0.87–3.81) were found in cropland soil. Our results suggest that afforestation could alter weight distribution of soil fractions and increase the soil organic C and N storage, thereby affecting the turnover and stability of SOM.

Impacts of Spartina alterniflora invasion on soil organic carbon and nitrogen pools sizes, stability, and turnover in a coastal salt marsh of eastern China

Plant invasion may impact ecosystem structure and function, and further affect soil organic matter (SOM) dynamics. However, the influence of plant invasion on soil organic carbon (C) and nitrogen (N) pools sizes, stability, and turnover in SOM of invaded ecosystems is not fully understood. In this study, soil C and N contents, and δ13C and δ15N values of free light fraction (LF), intra-aggregate particulate organic matter (iPOM) and mineral-associated organic matter (mSOM) were investigated in an invasive Spartina alterniflora community, adjacent bare flat and native Suaeda salsa and Phragmites australis communities. Short-term S. alterniflora invasion significantly enhanced organic C and N contents in SOM, free LF, iPOM, mSOM compared with bare flat and increased the proportion of allocated C in iPOM compared with S. salsa and P. australis soils (0–0.30m depth). The proportion of the S. alterniflora-derived C in free LF and iPOM were significantly higher than that in mSOM, and the highest S. alterniflora-derived C content was found in iPOM of S. alterniflora soil. The most enriched δ15N values were found in S. alterniflora soil. Increased δ15N values and decreased C:N ratios from the free LF to iPOM to mSOM in S. alterniflora soil indicated a greater degree of decomposition. The results suggest that 10-year S. alterniflora invasion significantly alters soil organic C and N pools sizes and stability through changing plant residuals input, physical distribution of S. alterniflora-derived C and C turnover in SOM fractions.

Long-term soil organic carbon and nitrogen dynamics after conversion of tropical forest to traditional sugarcane agriculture in East Mexico

Conversion of tropical forests to croplands greatly reduces stock size of soil organic resources. Low-impact agriculture may curtail this loss in soil fertility, and favor long-term agroecosystem functioning. In east tropical Mexico, indigenous smallholders have been managing sugarcane plots without burning, fertilizing, tillaging, and removal of crop residues for the last 70 years. This study examined the long-term effects of such low-input sugarcane agriculture on soil aggregate structure and soil organic carbon (SOC) and soil nitrogen (N) stocks along a 50-year chronosequence of cultivation including differently aged sugarcane plots and tropical forests as a reference site. Soils were physically fractioned in four aggregate size classes and in four soil organic matter (SOM) fractions and changes in C concentrations and C/N ratios of these fractions were assessed. Long-term cultivation did not alter soil aggregate structure, or litter C content; however it reduced significantly litter N content in sugarcane compared to forest plots. After 20 years of cultivation, SOC and soil N stocks dropped by 25% at 0–10cm soil depth compared to current C stocks in forest soils. After 50 years of cultivation, pooled over 0–20cm, SOC stocks were similar in sugarcane and forest plots, while soil N stocks remained 12–19% lower in sugarcane than in forest plots. The mineral-associated organic C fraction remained unaffected by land use change. Forest conversion to sugarcane, depleted the free light particulate organic matter (POM) C fraction at 0–10cm depth. Forest intra-aggregate POM C concentration declined after 20 years of cultivation and then recovered after 50 years of cultivation at 0–10cm depth. This study demonstrates that traditional sugarcane cropping may be a long-term sustainable alternative to sequester SOC, while maintaining moderate soil N fertility levels without trading off on sugarcane production.

Transition from intensive tillage to no-till enhances carbon sequestration in microaggregates of surface soil in the North China Plain

Physical protection by soil aggregates is critical for building soil organic carbon (SOC) stock. The objective of this study was to identify SOC sequestrated in the microaggregate holding within macroaggregte (mM) fraction after shifting tillage systems in the North China Plain. Soil samples from 0–5cm layer of a 6-yr field experiment (MP−R, moldboard plow without residue; MP+R, moldboard plow with residue; RT, rotary tillage with residue; NT, no-till with residue) were collected and separated into different water-stable aggregates. The macroaggregate (250–2000μm) was further isolated into intra-aggregate particulate organic matter (iPOM) fractions by density flotation, dispersion and sieving. The results showed that the SOC concentration of fine iPOM (250f, 53–250μm) was increased by 23% in RT and 39% in NT compared with MP+R, whereas the difference in the coarse iPOM (250c, >250μm) was not observed. The ratio of 250f–250c (i.e., 250f/250c) followed the order of NT (2.12)≈RT (1.94)>MP+R (1.50)≈MP−R (1.47), indicating the alternative tillage systems decreased the turnover rates of macroaggregates. Adoption of NT and RT improved the mM formation by 36% and 23% and mM associated C concentration by 38% and 31% as relative to MP+R system. Additionally, the soil C concentration and storage of the iPOM and silt plus clay fractions located within the microaggregate were higher under NT and RT than that of MP+R and MP−R systems. Thus applying NT and RT improved mM formation and soil C sequestered inside this fraction. We concluded that adoption of NT and RT enhanced SOC sequestration in the microaggregates of surface soil of the intensive agroecosystem of North China.

Dynamics of Soil Organic Carbon Fractions and Aggregates in Vegetable Cropping Systems

Fertilisers significantly affect crop production and crop biomass inputs to soil organic carbon (SOC). However, the long-term effects of fertilisers on C associated with aggregates are not yet fully understood. Based on soil aggregate and SOC fractionation analysis, this study investigated the long-term effects of organic manure and inorganic fertilisers on the accumulation and change in SOC and its fractions, including the C concentrations of free light fraction, intra-aggregate particulate organic matter (POM) and intra-aggregate mineral-associated organic matter (MOM). Long-term manure applications improved SOC and increased the concentrations of some C fractions. Manure also accelerated the decomposition of coarse POM (cPOM) into fine POM (fPOM) and facilitated the transformation of fPOM encrustation into intra-microaggregate POM within macroaggregates. However, the application of inorganic fertilisers was detrimental to the formation of fPOM and to the subsequent encrustation of fPOM with clay particles, thus inhibiting the formation of stable microaggregates within macroaggregates. No significant differences were observed among the inorganic fertiliser treatments in terms of C concentrations of MOM, intra-microaggregate MOM within macroaggregate (imMMOM) and intra-microaggregate MOM (imMOM). However, the long-term application of manure resulted in large increases in C concentrations of MOM (36.35%), imMMOM (456.31%) and imMOM (19.33%) compared with control treatment.

A long‐term nitrogen fertilizer gradient has little effect on soil organic matter in a high‐intensity maize production system

Global maize production alters an enormous soil organic C (SOC) stock, ultimately affecting greenhouse gas concentrations and the capacity of agroecosystems to buffer climate variability. Inorganic N fertilizer is perhaps the most important factor affecting SOC within maize-based systems due to its effects on crop residue production and SOC mineralization. Using a continuous maize cropping system with a 13 year N fertilizer gradient (0-269 kg N ha(-1) yr(-1)) that created a large range in crop residue inputs (3.60-9.94 Mg dry matter ha(-1) yr(-1)), we provide the first agronomic assessment of long-term N fertilizer effects on SOC with direct reference to N rates that are empirically determined to be insufficient, optimum, and excessive. Across the N fertilizer gradient, SOC in physico-chemically protected pools was not affected by N fertilizer rate or residue inputs. However, unprotected particulate organic matter (POM) fractions increased with residue inputs. Although N fertilizer was negatively linearly correlated with POM C/N ratios, the slope of this relationship decreased from the least decomposed POM pools (coarse POM) to the most decomposed POM pools (fine intra-aggregate POM). Moreover, C/N ratios of protected pools did not vary across N rates, suggesting little effect of N fertilizer on soil organic matter (SOM) after decomposition of POM. Comparing a N rate within 4% of agronomic optimum (208 kg N ha(-1) yr(-1)) and an excessive N rate (269 kg N ha(-1) yr(-1)), there were no differences between SOC amount, SOM C/N ratios, or microbial biomass and composition. These data suggest that excessive N fertilizer had little effect on SOM and they complement agronomic assessments of environmental N losses, that demonstrate N2 O and NO3 emissions exponentially increase when agronomic optimum N is surpassed.

Aggregate-associated soil organic matter as an ecosystem property and a measurement tool

Our 2000 paper Soil macroaggregate turnover and microaggregate formation: A mechanism for C sequestration under no-tillage agriculture had its genesis in attempts to identify and isolate soil organic matter (SOM) fractions that reflect the impacts of climate, soil physiochemical properties and physical disturbance on the soil organic carbon balance. A key prerequisite for the investigation was the development of a simple device to isolate the microaggregates (53–250 μm) contained within stable (i.e., resistant to slaking) macroaggregates (>250 μm) obtained by conventional wet-sieving. By comparing the abundance and C content of micro-within-macroaggregates, the size distribution of intra-aggregate particulate organic matter (iPOM) and isotopically-based estimates of the age of the organic matter in the different fractions, we were able to corroborate our hypothesis that the absence of tillage (i.e., in no-till and native soils) promotes greater longevity of newly-formed macroaggregates, resulting in greater SOM stabilization in microaggregates formed within stable macroaggregates. Follow-up research has indicated that the microaggregate-within-macroaggregate fraction is 1) potentially a robust indicator for management-induced SOC changes over decadal time scales, 2) of biological origin and therefore useful in interpreting impacts of soil biota on soil C and N dynamics, but not in-situ CO2 and N2O fluxes, 3) useful in complimentary chemical and spectroscopic approaches to relate SOM dynamics to soil structure and attributes of the soil pore space, and 4) a good candidate for being incorporated into models as a measurable fraction.

Soil organic carbon and total nitrogen in intensively managed arable soils

The conversion from cereal fields to vegetable production in the last three decades represents a significant shift in land use in China. Here, we studied the effects of conversion form cereal fields to vegetable production in north China on soil organic carbon (SOC) and total nitrogen (TN) in both bulk soil and soil aggregates. We used two approaches: (1) measurements of paired soil samples from wheat (Triticum aestivum L.) – maize (Zea mays L.) fields and adjacent greenhouses vegetable fields in three vegetable production areas representing various management intensities in terms of C and N inputs and frequency of tillage; (2) fractionating soil to distinguish intra-aggregate particulate organic matter (iPOM) and organo-mineral complexes (silt+clay). Our results indicated that converting cereal fields to greenhouse vegetable production with intermediate and high management intensity led to increases in SOC and TN and decreases in C:N ratios in the top soil. The accumulation rates of C and N in the surface soil (0–30cm) were estimated to be 1.37MgCha−1yr−1 and 0.21MgNha−1yr−1 over an average period of 8 years after cereal fields to greenhouse vegetable production conversion. At the soil aggregate level, only the coarse (>250μm) and fine (53–250μm) iPOM fraction contributed to the increases in soil C (e.g., 49% and 51% of total C increases, respectively), while the coarse and fine iPOM, and silt+clay fraction accounted for 22%, 30% and 48%, respectively, of total N increases. This illustrates how the addition of readily available C (manure) and N (manure and inorganic N) leads to a temporary stabilization of C in relatively labile SOM fractions, but to a preferential stabilization of N in organo-mineral SOM fractions. In conclusion, the conversion to highly intensive vegetable systems in China leads to marked differences in C and N stabilization dynamics.