Aggregate-associated Soil Organic Matter Research Articles

Physical and chemical processes determining soil organic matter dynamics in a managed vertisol in a tropical dryland area

Vertisols are soils with a high potential for carbon (C) sequestration. However, these soils undergo swelling and shrinkage cycles, which exert a strong influence on soil organic matter (SOM) dynamics. The long-term effects of agricultural conservation practices on SOM content and stability on a Vertisol located in a tropical dryland area was evaluated.Two tillage systems (conventional tillage [CT], disc ploughing and extraction of 70% harvest residues, and no tillage [NT], incorporating 100% of the harvest residues) in combination with three different crop rotations (sorghum-soyabean [SrSy], maize-maize [MM] or soyabean-maize [SyM]) were tested. All plots were irrigated during dry periods. The effects on soil organic C, N, microbial biomass and aggregate size class distribution were evaluated. Carbon sink capacity were assessed by studying the aggregate-associated SOM content and quality (thermal recalcitrance), as determined by thermo-calorimetry.Relative to the CT system, NT yielded a higher percentage of small macroaggregates (2-0.25 mm). It also promoted the accumulation of aggregate-associated SOM, especially in the small macroaggregates and microaggregates. Thermal analysis revealed that the aggregate-associated SOM was more recalcitrant (higher values of T50 and more energy released at 475 °C) in NT than in CT. Coinciding with these effects, the soil microbial biomass/SOC ratio was lower in NT than in CT. The gain of SOM and the higher recalcitrance of OM in the NT soils may be related to different characteristics of NT Vertisols, such as poor aeration or disruption of the swelling and shrinkage cycles. The findings show that soil conservation practices increase the capacity for long-term C sequestration in Vertisols via organomineral interactions and SOM recalcitrance, which are determined by the formation of large cracks as a result of the swelling and shrinkage cycles that are some of the specific properties and processes of these soils.

Carbon and nutrient mineralisation dynamics in aggregate-size classes from different tillage systems after input of canola and wheat residues

Aggregate-size classes may have different microbial accessibility and therefore different decomposability of aggregate-associated soil organic matter (SOM). However, processes and mechanisms of soil organic carbon (SOC) mineralisation and availability of nutrients [nitrogen (N), phosphorus (P) and sulphur (S)] in different aggregate-size classes, and particularly, the interaction of aggregates with tillage intensity and crop residue type in contrasting soils is poorly understood. Soil samples from conventional tillage (CT) and reduced tillage (RT) systems under mixed wheat–pasture farming, and no-till (NT) under continuous cereal–cover cropping in a Luvisol, and from CT and NT under continuous wheat cropping in a Vertisol, were separated into three dry aggregate classes of different sizes [mega-aggregates (>2–6.5 mm), macro-aggregates (0.25–2 mm) and micro-aggregates (<0.25 mm)]. Two residue types (canola and wheat stem; δ13C 124 and 460‰, respectively) were added into each of the three aggregate class samples from Luvisol (δ13C −24.7‰) and Vertisol (δ13C −18.5‰). Total CO2-C, δ13C in CO2-C, microbial biomass C (MBC), and plant available N, P and S were measured periodically during the 126-day incubation. The results showed that crop residue input increased native SOC mineralisation (via positive priming), MBC and microbial metabolic quotient in all three aggregate-size classes from different tillage systems in both soils. Native SOC mineralisation was 1.5–3.7 and 0.6–2.8 times higher in the canola and wheat residue-amended (cf. control, non-amended) aggregates, respectively. Native SOC mineralisatiion and MBC were higher in the macro- and micro- than mega-aggregates in both soils. However, priming of native SOC mineralisation, relative to the control, was similar across the aggregates, except for the CT in the Luvisol where priming was higher in the macro- than micro- and mega-aggregates. Native SOC mineralisation among the aggregate-size classes was 26–114% higher under CT or RT cf. NT in the Luvisol but was similar under CT and NT in the Vertisol. Net available N was significantly higher in the residue-amended than the control aggregates, particularly in the CT and/or RT versus the NT at day 30 only, and mainly in the Luvisol. Further, substantial amounts of available P and S were released from the residue-amended versus the control aggregates at day 126, with Vertisol releasing 2–3 times more available P than Luvisol. In conclusion, our findings showed the importance of returning crop residues to enhance nutrient availability from all aggregate-size classes in different soils and farming systems. In particular, the tillage (versus no-till) and canola (versus wheat) residue induced a greater release of nutrients, generally in the pattern of micro- ≥ macro- > mega-aggregates. Clearly, the input of crop residues enhanced the release of SOM-bound nutrients, possibly via positive priming, and may have mobilised mineral-bound nutrients, such as P and S in each aggregate-size class, with tillage intensity and soil type modulating these processes.

Crop residue retention enhances soil properties and nitrogen cycling in smallholder maize systems of Chiapas, Mexico

Conservation agriculture (CA) encompasses three principles: reduced tillage, residue retention, and crop rotation, and is thought to hold great potential for improving soil function and maintaining long-term productivity in agricultural systems. However, the benefits of CA likely depend on the biophysical and socioeconomic context in which CA technologies are implemented and whether or not all components of CA are in place. To better understand the effect of key CA components on soil quality and nutrient use efficiency, an on-farm field trial was conducted during the second year of an irrigated field experiment in Chiapas, Mexico. The experiment included two levels each of tillage and residue management in a full factorial design (three replicates each) with the following treatments: 1) conventional tillage with crop residue; 2) conventional tillage without crop residue; 3) zero tillage with crop residue; and 4) zero tillage without crop residue. In each replicate plot (8×20m) a 15N labeled microplot (0.95×0.93m) was established and fertilized according to standard rates. Mid-season soil samples were analyzed for total and available (Bray) P, as well as microbial community composition using phospholipid fatty acid (PLFA) analysis. Crop yield, nutrient uptake, and aggregate-associated soil organic matter were evaluated at harvest. Crop residue retention led to small increases in aggregate stability and greater fungal PLFA. The retention of crop residues also resulted in a 110% increase in the recovery of fertilizer-derived N in vegetative biomass, leading to a marginally significant increase of 41% for total recovery of fertilizer N in the soil-crop system. Effects of tillage were less prominent, but conventional tillage was found to improve mid-season available P by roughly 60%. These results indicate that CA technologies, residue retention in particular, offer a promising set of options for farmers to improve soil functioning and reduce fertilizer-N losses, particularly for sandy soils within sub-humid tropical regions.

Six years of different tillage systems affected aggregate-associated SOM in a semi-arid loam soil from Central Iran

Tillage practices potentially influence soil organic matter (SOM) storage and dynamics, which is important for sustainable cropping practices and agricultural productivity, and carbon (C) sequestration in soil. The objective of the current study was to quantify changes in bulk SOM and aggregate-associated SOM in a loam soil following four tillage practices for a period of six years (2005–2011). The amount of bulk soil organic C and nitrogen (N), aggregate stability, and C and N associated with both macroaggregates (4–0.250mm) and microaggregates (0.250–0.053mm) at the 0–20cm depth were evaluated for a semi-arid loam soil under moldboard (MP), disk (DP), chisel (CP) and rotary (RP) plows with a similar addition of organic residues to the soil under all the tillage systems. The experiment was designed as a randomized complete block with three replicates of each tillage treatment. Soil samples were obtained during the experiment in 2008, 2009 and 2011. SOM (C and N) pool did not vary between tillage treatments, while the pools of C and N averaged across tillage treatments were increased with time. This suggests that changes in SOM were more evident with addition of plant residues than with tillage systems. Soil aggregate stability indices (i.e., mean weight diameter and aggregate ratio values) were significantly greater in CP and RP than MP and DP practices in all the sampling years mainly due to an increase of the macroaggregate fraction, and concurrently a decline of the microaggregate fraction. Similarly, soil aggregate stability indices were increased with increasing the time period of experiment in all the tillage systems and when compared with the initial values before the experiment. CP and RP practices increased the pools of C (10–11%) and N (13–15%) associated with the macroaggregate-size fraction, while decreasing C (50–66%) and N (48–61%) associated with the microaggregate-size fraction. Furthermore, differences in SOM and aggregate stability between MP and DP practices and between CP and RP practices were not statistically significant. It is concluded that a reduction in tillage intensity under both CP and RP would not improve total SOM in the short-term, but would improve soil structure and shift SOM from the microaggregate to macroaggregate fractions without a substantial change in the whole SOM of the studied calcareous soil. Soil aggregate stability and SOM associated with aggregate fractions appear to be more sensitive to short-term tillage practices than the whole SOM in this semiarid soil.

Pasture degradation impacts soil phosphorus storage via changes to aggregate-associated soil organic matter in highly weathered tropical soils

Maintaining the productivity of tropical pastures is a major challenge for the sustainable management of tropical landscapes around the globe. To address this issue, we examined linkages between soil organic matter (SOM), aggregation, and phosphorus (P) dynamics by comparing productive vs. degraded pastures in the deforested Amazon Basin of Colombia. Paired plots of productive (dominated by planted Brachiaria spp.) vs. degraded pasture were identified on nine farms in the Department of Caquetá and sampled during the rainy season of 2011. Aboveground pasture biomass production and nutrient content were measured. Surface soils (0–10 cm) were also fractionated by wet sieving, and C, 13C, N and P contents were analyzed for the bulk soil and various aggregate size classes. Productive pastures yielded more than double the aboveground biomass compared to degraded pastures (during a 35 day regrowth period following cutting), with over 60% higher N and P contents in this material. Similar trends were observed for the standing litter biomass and nutrient contents. Soil aggregate stability was found to differ between pasture types, with a mean weight diameter of 3590 vs. 3230 μm in productive vs. degraded pastures, respectively. Productive pastures were found to have 20% higher total soil C and N contents than degraded pastures. While there was no difference in total P content between pasture types, organic P was found to be nearly 40% higher in soils of productive vs. degraded pastures. Differences in total SOM between pasture types were largely explained by a higher C content in the large macroaggregate fraction (>2000 μm), and more specifically in the microaggregates (53–250 μm) occluded within this fraction. These findings confirm the role of microaggregates within macroaggregates as a preferential site for the physical stabilization of SOM, and furthermore, suggest that it may serve as a useful diagnostic fraction for evaluating management impacts on SOM in tropical pasture systems. Similar to trends observed for C and N, total P content was 25% higher in the microaggregates within large macroaggregates of productive vs. degraded pasture soils. This correspondence between C and total P contents in large macroaggregate fractions, along with elevated levels of organic P in productive pastures, suggests that this P is likely in an organic form and that there is a close link between soil structure, SOM dynamics and the maintenance of organic P in these soils. Given the potential relevance of organic P for efficient P cycling in these soils, our findings offer critical new insight for the management of SOM and aggregate-associated P pools in tropical pasture systems.

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.

Earthworms and litter management contributions to ecosystem services in a tropical agroforestry system

The development of sustainable agricultural systems depends in part upon improved management of non-crop species to enhance the overall functioning and provision of services by agroecosystems. To address this need, our research examined the role of earthworms and litter management on nutrient dynamics, soil organic matter (SOM) stabilization, and crop growth in the Quesungual agroforestry system of western Honduras. Field mesocosms were established with two earthworm treatments (0 vs. 8 Pontoscolex corethrurus individuals per mesocosm) and four litter quality treatments: (1) low-quality Zea mays, (2) high-quality Diphysa robinioides, (3) a mixture of low- and high-quality litters, and (4) a control with no organic residues applied. Mesocosms included a single Z. mays plant and additions of 15N-labeled inorganic nitrogen. At maize harvest, surface soils (0-15 cm) in the mesocosms were sampled to determine total and available P as well as the distribution of C, N, and 15N among different aggregate-associated SOM pools. Maize plants were divided into grain and non-grain components and analyzed for total P, N, and 15N. Earthworm additions improved soil structure as demonstrated by a 10% increase in mean weight diameter and higher C and N storage within large macro-aggregates (>2000 microm). A corresponding 17% increase in C contained in micro-aggregates within the macro-aggregates indicates that earthworms enhance the stabilization of SOM in these soils; however, this effect only occurred when organic residues were applied. Earthworms also decreased available P and total soil P, indicating that earthworms may facilitate the loss of labile P added to this system. Earthworms decreased the recovery of fertilizer-derived N in the soil but increased the uptake of 15N by maize by 7%. Litter treatments yielded minimal effects on soil properties and plant growth. Our results indicate that the application of litter inputs and proper management of earthworm populations can have important implications for the provision of ecosystem services (e.g., C sequestration, soil fertility, and plant production) by tropical agroforestry systems.

Earthworm impacts on soil organic matter and fertilizer dynamics in tropical hillside agroecosystems of Honduras

Abstract Earthworms are important processors of soil organic matter (SOM) and nutrient turnover in terrestrial ecosystems. In agroecosystems, they are often seen as beneficial organisms to crop growth and are actively promoted by farmers and extension agents, yet their contribution to agroecosystem services is uncertain and depends largely on management. The Quesungual slash-and-mulch agroforestry system (QSMAS) of western Honduras has been proposed as a viable alternative to traditional slash-and-burn (SB) practices and has been shown to increase earthworm populations, yet the effect of earthworms on soil fertility and SOM in QSMAS is poorly understood. This study examined the role of Pontoscolex corethrurus in QSMAS by comparing their influence on aggregate-associated SOM and fertilizer dynamics with their effects under SB and secondary forest in a replicated field trial. Both the fertilized QSMAS and SB treatments had plots receiving additions of inorganic 15N and P, as well as plots with no inorganic N additions. Earthworm populations were manipulated in field microcosms at the beginning of the rainy season within each management treatment via additions of P. corethrurus or complete removal of existing earthworm populations. Microcosms were destructively sampled at harvest of Zea mays and soils were wet-sieved (using 53, 250 and 2000 μm mesh sizes) to isolate different aggregate size fractions, which were analyzed for total C, N and 15N. The effects of management system were smaller than expected, likely due to disturbance associated with the microcosm installation. Contrary to our hypothesis that earthworms would stabilize organic matter in soil aggregates, P. corethrurus decreased total soil C by 3% in the surface layer (0–15 cm), predominantly through a decrease in the C concentration of macroaggregates (>250 μm) and a corresponding depletion of C in coarse particulate organic matter occluded within macroaggregates. Earthworms also decreased bulk density by over 4%, but had no effect on aggregate size distribution. Within the two fertilized treatments, the QSMAS appeared to retain slightly more fertilizer derived N in smaller aggregate fractions (

Earthworms, soil fertility and aggregate-associated soil organic matter dynamics in the Quesungual agroforestry system

Issues of food security, environmental degradation and global climate change underscore the need for the improved understanding of sustainable agricultural systems around the globe. The Quesungual slash-and-mulch agroforestry system (QSMAS) of western Honduras offers a promising alternative to traditional slash-and-burn (SB) agriculture for the mountainous tropical dry forest zones of Central America, but the overall influence of this system on soils is not fully understood. We examined earthworm populations, soil fertility and soil organic matter (SOM) dynamics under QSMAS and SB agriculture, with secondary forest (SF) as a reference. Both QSMAS and SB consisted of treatments with and without inorganic fertilizer (N–P–K) additions, resulting in five management treatments, each present on three replicate farms. Baseline soil samples (0–15 cm) were collected prior to forest clearing and establishment of QSMAS plots in 2003 and in SB and SF plots in 2005 to determine initial soil concentrations of C and N. Soils were sampled in 2006 and 2007 for bulk soil C and N and P availability, as well as for aggregate fractionation and determination of C and N within the different aggregate size fractions. Earthworm populations were assessed in July 2007. Earthworm numbers and biomass were higher under QSMAS than under SB (13.4 vs. 0.8 g fresh biomass m − 2 ; respectively). Significant interactions between cropping system and fertilization suggest that QSMAS increased the availability of added inorganic P, 3 times more under QSMAS than for SB. Comparisons with SF, indicated that both cropping systems resulted in a dramatic loss of C (average 5 g C kg − 1 soil) since treatment implementation, and that this loss was mainly associated with the disruption of C rich large macroaggregates (> 2000 µm). After taking into account baseline soil C differences between plots, no major differences in total SOM losses were found between QSMAS and SB management. However, earlier establishment of QSMAS plots suggests that the overall rate of C loss since treatment establishment was lower for QSMAS than for SB. Results from this study suggest that the Quesungual agroforestry system offers great potential to improve soil fertility and biological health in the region relative to traditional slash-and-burn agriculture.