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DisciplinesAgricultural Science | Agriculture | Agronomy and Crop Sciences CommentsLabile, 'high-quality' , plant litters are hypothesized to promote soil organic matter (SOM) stabilization in mineral soil fractions that are physicochemically protected from rapid mineralization. However, the effect of litter quality on SOM stabilization is inconsistent. High-quality litters, characterized by high N concentrations, low C/N ratios, and low phenol/lignin concentrations, are not consistently stabilized in SOM with greater efficiency than 'low-quality' litters characterized by low N concentrations, high C/N ratios, and high phenol/lignin concentrations. Here, we attempt to resolve these inconsistent results by developing a new conceptual model that links litter quality to the soil C saturation concept. Our model builds on the Microbial Efficiency-Matrix Stabilization framework (Cotrufo et al., 2013) by suggesting the effect of litter quality on SOM stabilization is modulated by the extent of soil C saturation such that high-quality litters are not always stabilized in SOM with greater efficiency than low-quality litters. RightsWorks produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The content of this document is not copyrighted.This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/agron_pubs/100Integrating plant litter quality, soil organic matter stabilization, and the carbon saturation concept AbstractLabile, 'high-quality', plant litters are hypothesized to promote soil organic matter (SOM) stabilization in mineral soil fractions that are physicochemically protected from rapid mineralization. However, the effect of litter quality on SOM stabilization is inconsistent. High-quality litters, characterized by high N concentrations, low C/N ratios, and low phenol/lignin concentrations, are not consistently stabilized in SOM with greater efficiency than 'low-quality' litters characterized by low N concentrations, high C/N ratios, and high phenol/lignin concentrations. Here, we attempt to resolve these inconsistent results by developing a new conceptual model that links litter quality to the soil C saturation concept. Our model builds on the Microbial Efficiency-Matrix Stabilization framework (Cotrufo et al., 2013) by suggesting the effect of litter quality on SOM stabilization is modulated by the extent of soil C saturation such that high-quality litters are not always stabilized in SOM with greater efficiency than low-quality litters.

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To study the structure and function of soil organic matter, soil scientists have performed alkali extractions for soil humic acid (HA) and fulvic acid (FA) fractions for more than 200 years. Over the last few decades aquatic scientists have used similar fractions of dissolved organic matter, extracted by resin adsorption followed by alkali desorption. Critics have claimed that alkaliextractable fractions are laboratory artifacts, hence unsuitable for studying natural organic matter structure and function in field conditions. In response, this review first addresses specific conceptual concerns about humic fractions. Then we discuss several case studies in which HA and FA were extracted from soils, waters, and organic materials to address meaningful problems across diverse research settings. Specifically, one case study demonstrated the importance of humic substances for understanding transport and bioavailability of persistent organic pollutants. An understanding of metal binding sites in FA and HA proved essential to accurately model metal ion behavior in soil and water. In landscape-based studies, pesticides were preferentially bound to HA, reducing their mobility. Compost maturity and acceptability of other organic waste for land application were well evaluated by properties of HA extracted from these materials. A young humic fraction helped understand N cycling in paddy rice (Oryza sativa L.) soils, leading to improved rice management. The HA and FA fractions accurately represent natural organic matter across multiple environments, source materials, and research objectives. Studying them can help resolve important scientific and practical issues.

Rice systems in Asia have intensified rapidly in the past 30 years, and significant areas of irrigated lowland rice are now supporting two or three rice crops per year. Our objective was to compare the chemical composition of soil organic matter (SOM) from four fields with different histories of rice cropping intensity and soil submergence: (i) a single-crop rainfed, dryland rice system without soil submergence, (ii) an irrigated rice and soybean rotation, and irrigated (iii) double-or (iv) triplecrop rice systems in which soil remains submerged during much of the year. In all four soils, extracted mobile humic acid (MHA) and calcium humate (CaHA) fractions were of modem age by I4C-dating, and represented about 20% of total N and organic C. The MHA was enriched in N and hydrolysable amino acids (AA) compared with CaHA in all soils. With increased frequency of irrigated rice cropping, however, there was a large increase in phenolic content of SOM. We speculate that slower lignin decomposition caused by deficiency of 0 2 in submerged soil leads to incorporation of phenolic moieties into young SOM fractions. The increased phenolic character of these fractions may influence N cycling and the N supplying capacity of lowland soils supporting two or three annual crops of irrigated rice.

Large-scale soil application of biochar may enhance soil fertility, increasing crop production for the growing human population, while also sequestering atmospheric carbon. But reaching these beneficial outcomes requires an understanding of the relationships among biochar's structure, stability, and contribution to soil fertility. Using quantitative 13 C nuclear magnetic resonance (NMR) spectroscopy, we show that Terra Preta soils (fertile anthropogenic dark earths in Amazonia that were enriched with char >800 years ago) consist predominantly of char residues composed of ∼6 fused aromatic rings substituted by COO − groups that significantly increase the soils' cation-exchange capacity and thus the retention of plant nutrients. We also show that highly productive, grassland-derived soils in the U.S. (Mollisols) contain char (generated by presettlement fires) that is structurally comparable to char in the Terra Preta soils and much more abundant than previously thought (∼40− 50% of organic C). Our findings indicate that these oxidized char residues represent a particularly stable, abundant, and fertilityenhancing form of soil organic matter.

Previous research has shown that long-term intensive cropping of irrigated lowland rice has led to significant grain-yield declines in field trials. The yield decline was attributed to decreased availability of soil nitrogen, which is held mostly in the soil organic matter. By advanced solid-state NMR spectroscopy, we have detected significant amounts of amide nitrogen directly bonded to aromatic rings in a humic acid fraction extracted from a continually submerged, triple-cropped rice soil. Because nitrogen bonded to aromatics is not readily plant-available, this observation can explain the yield decline. Quantitative 13 C NMR combined with advanced spectral editing showed that this humic acid is rich in lignin derivatives (>45% of all carbon), whereas the corresponding humic acid fraction extracted from an aerobic, single-cropped rice soil contains less lignin and less nitrogen bonded to aromatics. C ultivation of two or three rice crops annually in irrigated lowland soils has been the foundation of Asia's rice supply since such intensive cropping first became possible in the 1960s. Currently, approximately one quarter of global rice production comes from multiple annual cropping of lowland rice, and this central role in the food supply will expand in the future as Asian populations continue to increase. In long-term field trials in which initial yield levels of lowland rice approached the yieldpotential ceiling, yields declined by Ͼ35% during 20-30 years of double and triple cropping (1). The search for mitigation options that will reverse this yield decline and thus improve the food supply for a significant portion of the world's population requires an understanding of the underlying processes.Agronomic data indicate that this yield decline resulted primarily from decreased crop uptake of soil nitrogen (1), which is distinct from fertilizer nitrogen and is held mostly in the soil organic matter. However, total soil nitrogen did not decrease in quantity as yields declined. We propose that the apparent decrease in availability of soil nitrogen is caused by the chemical stabilization of nitrogenous compounds by bonding to aromatic rings in lignin residues, probably via phenolic functionalities. Lignin residues accumulate in soils that are intensively cropped to irrigated lowland rice (1) because of their slow decomposition under the characteristically anaerobic conditions and their high input rates through multiple incorporations annually of crop residues. Nitrogen directly bonded to or in aromatic rings has been found to be less bioavailable than nitrogen in peptides, based on its rate of degradation through soil microbial processes under laboratory conditions (2-4), but 15 N NMR spectroscopy has been unable to detect significant fractions of nitrogen bonded to aromatic carbons in the organic matter of these rice soils or any other soils (5), raising doubts as to whether nitrogen can be bonded to aromatic carbons under field conditions. Recent developments in solid-state NMR pulse sequences have greatly advanc...

Nitrogen fertilization is critical to optimize short-term crop yield, but its long-term effect on soil organic C (SOC) is uncertain. Here, we clarify the impact of N fertilization on SOC in typical maize-based (Zea mays L.) Midwest U.S. cropping systems by accounting for site-to-site variability in maize yield response to N fertilization. Within continuous maize and maize-soybean [Glycine max (L.) Merr.] systems at four Iowa locations, we evaluated changes in surface SOC over 14 to 16 years across a range of N fertilizer rates empirically determined to be insufficient, optimum, or excessive for maximum maize yield. Soil organic C balances were negative where no N was applied but neutral (maize-soybean) or positive (continuous maize) at the agronomic optimum N rate (AONR). For continuous maize, the rate of SOC storage increased with increasing N rate, reaching a maximum at the AONR and decreasing above the AONR. Greater SOC storage in the optimally fertilized continuous maize system than in the optimally fertilized maize-soybean system was attributed to greater crop residue production and greater SOC storage efficiency in the continuous maize system. Mean annual crop residue production at the AONR was 22% greater in the continuous maize system than in the maize-soybean system and the rate of SOC storage per unit residue C input was 58% greater in the monocrop system. Our results demonstrate that agronomic optimum N fertilization is critical to maintain or increase SOC of Midwest U.S. cropland.