Instead of containing stable and chemically unique ‘humic substances’, as has been widely accepted, soil organic matter is a mixture of progressively decomposing organic compounds; this has broad implications for soil science and its applications. The exchange of nutrients, energy and carbon between soil organic matter, the soil environment, aquatic systems and the atmosphere is important for agricultural productivity, water quality and climate. Long-standing theory suggests that soil organic matter is composed of inherently stable and chemically unique compounds. Here we argue that the available evidence does not support the formation of large-molecular-size and persistent ‘humic substances’ in soils. Instead, soil organic matter is a continuum of progressively decomposing organic compounds. We discuss implications of this view of the nature of soil organic matter for aquatic health, soil carbon–climate interactions and land management. Soil organic matter contains a large portion of the world's carbon and plays an important role in maintaining productive soils and water quality. Nevertheless, a consensus on the nature of soil organic matter is lacking. Johannes Lehmann and Markus Kleber argue that soil organic matter should no longer be seen as large and persistent, chemically unique substances, but as a continuum of progressively decomposing organic compounds.

The contentious nature of soil organic matter

https://hal.archives-ouvertes.fr/hal-01480573/document

Soil carbon 4 per mille

This Perspective looks at how microbial anabolism and the soil microbial carbon pump control microbial necromass accumulation and stabilization; the ‘entombing effect’.

The importance of anabolism in microbial control over soil carbon storage

In recent years, agricultural growth in China has accelerated remarkably, but most of this growth has been driven by increased yield per unit area rather than by expansion of the cultivated area Looking towards 2030, to meet the demand for grain and to feed a growing population on the available arable land, it is suggested that annual crop production should be increased to around 580 Mt and that yield should increase by at least 2% annually Crop production will become more difficult with climate change, resource scarcity (eg land, water, energy, and nutrients) and environmental degradation (eg declining soil quality, increased greenhouse gas emissions, and surface water eutrophication) To pursue the fastest and most practical route to improved yield, the near-term strategy is application and extension of existing agricultural technologies This would lead to substantial improvement in crop and soil management practices, which are currently suboptimal Two pivotal components are required if we are to follow new trajectories First, the disciplines of soil management and agronomy need to be given increased emphasis in research and teaching, as part of a grand food security challenge Second, continued genetic improvement in crop varieties will be vital However, our view is that the biggest gains from improved technology will come most immediately from combinations of improved crops and improved agronomical practices The objectives of this paper are to summarize the historical trend of crop production in China and to examine the main constraints to the further increase of crop productivity The paper provides a perspective on the challenge faced by science and technology in agriculture which must be met both in terms of increased crop productivity but also in increased resource use efficiency and the protection of environmental quality

Improving crop productivity and resource use efficiency to ensure food security and environmental quality in China

The impact of agricultural management on global warming potential (GWP) and greenhouse gas intensity (GHGI) is not well documented. A long-term fertilizer experiment in Chinese double rice-cropping systems initiated in 1990 was used in this study to gain an insight into a complete greenhouse gas accounting of GWP and GHGI. The six fertilizer treatments included inorganic fertilizer [nitrogen and phosphorus fertilizer (NP), nitrogen and potassium fertilizer (NK), and balanced inorganic fertilizer (NPK)], combined inorganic/organic fertilizers at full and reduced rate (FOM and ROM), and no fertilizer application as a control. Methane (CH4) and nitrous oxide (N2O) fluxes were measured using static chamber method from November 2006 through October 2009, and the net ecosystem carbon balance was estimated by the changes in topsoil (0–20 cm) organic carbon (SOC) density over the 10-year period 1999–2009. Long-term fertilizer application significantly increased grain yields, except for no difference between the NK and control plots. Annual topsoil SOC sequestration rate was estimated to be 0.96 t C ha−1 yr−1 for the control and 1.01–1.43 t C ha−1 yr−1 for the fertilizer plots. Long-term inorganic fertilizer application tended to increase CH4 emissions during the flooded rice season and significantly increased N2O emissions from drained soils during the nonrice season. Annual mean CH4 emissions ranged from 621 kg CH4 ha−1 for the control to 1175 kg CH4 ha−1 for the FOM plots, 63–83% of which derived from the late-rice season. Annual N2O emission averaged 1.15–4.11 kg N2O–N ha−1 in the double rice-cropping systems. Compared with the control, inorganic fertilizer application slightly increased the net annual GWPs, while they were remarkably increased by combined inorganic/organic fertilizer application. The GHGI was lowest for the NP and NPK plots and highest for the FOM and ROM plots. The results of this study suggest that agricultural economic viability and GHGs mitigation can be simultaneously achieved by balanced fertilizer application.

Net annual global warming potential and greenhouse gas intensity in Chinese double rice-cropping systems: a 3-year field measurement in long-term fertilizer experiments

The association of organic matter (OM) with minerals is recognized as the most important stabilization mechanism for soil organic matter. This study compared the properties of Fe–OM complexes formed from adsorption (reaction of OM to postsynthesis ferrihydrite) versus coprecipitation (formation of Fe solids in the presence of OM). Coprecipitates and adsorption complexes were synthesized using dissolved organic matter (DOM) extracts from a forest little layer at varying molar C/Fe ratios of 0.3–25.0. Sample properties were studied by N2 gas adsorption, XRD, FTIR, Fe EXAFS, and STXM-NEXAFS techniques. Coprecipitation resulted in much higher maximum C contents (∼130 mg g–1 C difference) in the solid products than adsorption, which may be related to the formation of precipitated insoluble Fe(III)–organic complexes at high C/Fe ratios in the coprecipitates as revealed by Fe EXAFS analysis. Coprecipitation led to a complete blockage of mineral surface sites and pores with ≥177 mg g–1 C and molar C/Fe ratios ≥2....

Properties of Fe-Organic Matter Associations via Coprecipitation versus Adsorption

The estimation of the size and changes of soil organic carbon (SOC) stocks is of great importance for decision makers to adopt proper measures to protect soils and to develop strategies for mitigation of greenhouse gases. In this paper, soil data from the Second State Soil Survey of China (SSSSC) conducted in the early 1980s and data published in the last 5 years were used to estimate the size of SOC stocks over the whole profile and their changes in China in last 20 years. Soils were identified as paddy, upland, forest, grassland or waste-land soils and an improved soil bulk density estimation method was used to estimate missing bulk density data. In the early 1980s, total SOC stocks were estimated at 89.61 Pg (1 Pg=103 Tg=1015 g) in China's 870.94 Mha terrestrial areas covered by 2473 soil series. In the paddy, upland, forest and grassland soils the respective total SOC stocks were 2.91 Pg on 29.87 Mha, 10.07 Pg on 125.89 Mha, 34.23 Pg on 249.32 Mha and 37.71 Pg on 278.51 Mha, respectively. The SOC density of the surface layer ranged from 3.5 Mg ha−1 in Gray Desery grassland soils to 252.6 Mg ha−1 in Mountain Meadow forest soils. The average area-weighted total SOC density in paddy soils (97.6 Mg ha−1) was higher than that in upland soils (80 Mg ha−1). Soils under forest (137.3 Mg ha−1) had a similar average area-weighted total SOC density as those under grassland (135.4 Mg ha−1). The annual estimated SOC accumulation rates in farmland and forest soils in the last 20 years were 23.61 and 11.72 Tg, respectively, leading to increases of 0.472 and 0.234 Pg SOC in farmland and forest areas, respectively. In contrast, SOC under grassland declined by 3.56 Pg due to the grassland degradation over this period. The resulting estimated net SOC loss in China's soils over the last 20 years was 2.86 Pg. The documented SOC accumulation in farmland and forest soils could thus not compensate for the loss of SOC in grassland soils in the last 20 years. There were, however, large regional differences: Soils in China's South and Eastern parts acted mainly as C sinks, increasing their average topsoil SOC by 132 and 145 Tg, respectively. In contrast, in the Northwest, Northeast, Inner Mongolia and Tibet significant losses of 1.38, 0.21, 0.49 and 1.01 Pg of SOC, respectively, were estimated over the last 20 years. These results highlight the importance to take measures to protect grassland and to improve management practices to increase C sequestration in farmland and forest soils.

Soil organic carbon stocks in China and changes from 1980s to 2000s

Aqueous Fe(II) is known to catalyze the abiotic transformation of ferrihydrite to more stable Fe minerals. However, little is known about the impacts of coprecipitated OM on Fe(II)-catalyzed ferrihydrite transformation and its consequences for C dynamics. Accordingly, we investigated the extent and pathway of Fe(II)-induced transformation of OM-ferrihydrite coprecipitates as a function of C/Fe ratios and aqueous Fe(II) concentrations, and its implications for subsequent C dynamics. The coprecipitated OM resulted in a linear decrease in ferrihydrite transformation with increasing C/Fe ratios. The secondary mineral profiles upon Fe(II) reaction with OM-ferrihydrite coprecipitates depend on Fe(II) concentrations At 0.2 mM Fe(II), OM completely inhibited goethite formation and stimulated lepidocrocite formation. At 2 mM Fe(II), whereas goethite was formed in the presence of OM, OM reduced the amount of goethite and magnetite formation and increased the formation of lepidocrocite. The solid-phase C content rem...

Influence of Coprecipitated Organic Matter on Fe2+(aq)-Catalyzed Transformation of Ferrihydrite: Implications for Carbon Dynamics.

Soil organic matter (SOM) is correlated with reactive iron (Fe) in humid soils, but Fe also promotes SOM decomposition when oxygen (O2) becomes limited. Here we quantify Fe-mediated OM protection vs. decomposition by adding 13C dissolved organic matter (DOM) and 57FeII to soil slurries incubated under static or fluctuating O2. We find Fe uniformly protects OM only under static oxic conditions, and only when Fe and DOM are added together: de novo reactive FeIII phases suppress DOM and SOM mineralization by 35 and 47%, respectively. Conversely, adding 57FeII alone increases SOM mineralization by 8% following oxidation to 57FeIII. Under O2 limitation, de novo reactive 57FeIII phases are preferentially reduced, increasing anaerobic mineralization of DOM and SOM by 74% and 32‒41%, respectively. Periodic O2 limitation is common in humid soils, so Fe does not intrinsically protect OM; rather reactive Fe phases require their own physiochemical protection to contribute to OM persistence.

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Iron-mediated organic matter decomposition in humid soils can counteract protection.

Carbon-mineral association between iron oxides and dissolved organic carbon (DOC) occurs ubiquitously in natural environments, and is thus an important control on the transport and sequestration of organic carbon. However, the stability of C-mineral association under reducing conditions and its implication on carbon turnover and cycling is unclear. In this study, anaerobic incubation experiments were conducted to investigate the biodegradability of ferrihydrite (Fe-oxide)-adsorbed natural DOC with known Fe-reducing bacterium, Shewanella oneidensis MR-1. Two types of bulk DOC were used to form the carbon-ferrihydrite association –forest fresh leaf layer (LDOC) and forest humified layer (HDOC). Under controlled pH and redox condition, DOC concentrations increased by 11.2 (±0.4)% and 13.3 (±0.3)% for LDOC and HDOC after anaerobic microbial iron reduction. Coupled to the DOC release, 28% and 14% of Fe(III) were reduced with LDOC and HDOC, respectively. Our results demonstrated that under controlled pH conditions, the C release was driven by dissimilatory iron reduction. Likely microbial iron reduction decrease the carbon sequestration potential via reducing the Fe-oxides surface area and therefore release DOC to the solution and make it more susceptible to microbial degradation. Further, dissimilatory iron reduction also shifted DOC spectroscopic properties (UV and fluorescence) and pronounced increase in humification index (HIX) values was observed after the incubation. Our results suggest dissimilatory iron reduction is an important mechanism for Fe dissolution and C mobilization, which impacts the long-term carbon transformation, storage and turnover in soil environments.

Chunmei Chen

Papers

Properties of Fe-Organic Matter Associations via Coprecipitation versus Adsorption

Soil organic carbon stocks in China and changes from 1980s to 2000s

Influence of Coprecipitated Organic Matter on Fe2+(aq)-Catalyzed Transformation of Ferrihydrite: Implications for Carbon Dynamics.

Iron-mediated organic matter decomposition in humid soils can counteract protection.

Dissimilatory microbial iron reduction release DOC (dissolved organic carbon) from carbon-ferrihydrite association