Nitrogen-bonded aromatics in soil organic matter and their implications for a yield decline in intensive rice cropping

Abstract

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...

Role of extractable and residual organic matter fractions on sorption of phenanthrene in sediments

“…Another possibility for the high extraction yield of lignin was the different extraction method, and other acid-hydrolyzable material besides lignin being extracted in the isolation. The latter possibility seems to be supported by the occurrence of N-containing compounds (average N $ 3.2%) in the LG isolates, which were supposed to contain no N. However, for the degraded lignin, amid functional groups can be added to the aromatic carbon in lignin during humification (Schmidt-Rohr et al, 2004). Moreover, the contents of C and O in the LG isolates were much close to the values for the reported lignin in the other studies, while H contents were slightly higher than their values (Xing et al, 1994;Chefetz et al, 2000;Wang et al, 2007b;Yang et al, 2010).…”

Chemical stabilization of soil organic nitrogen by phenolic lignin residues in anaerobic agroecosystems

“…The excess quantity of anilide-N (55 kg N/ha) in this fraction compared to the MHA fraction of a nearby aerated rice soil was similar to the historical decrease in crop uptake of soil N (60 kg N/ha season) that was associated with the yield decline. Hence the slower mineralization of anilide N could contribute substantially to the decreased availability of soil N. Schmidt-Rohr et al (2004) concluded that anilide N formed from phenolic lignin residues, based on the chemical properties of the aromatic portion of the anilide and because the abundance of anilide in three SOM fractions was proportional to the abundance of lignin residues. Their findings represent the first direct evidence for covalent binding of N by phenolic lignin residues in naturally formed SOM.…”

“…Recent studies have identified the accumulation of phenolic compounds in the SOM of submerged soils that were intensively cropped to irrigated lowland rice (Oryza sativa L.) in the Philippines (Olk et al, 1996(Olk et al, , 1998. Using newly developed analytical techniques to identify the bonding environments of C with N, phenolic lignin residues were shown to have bound covalently with N in a humic acid fraction (Schmidt-Rohr et al, 2004 resulting chemical stabilization was hypothesized to have contributed to an observed long-term decrease in availability of soil N and an associated decline in rice grain yield. In this review we summarize the concepts gained from this study of continuous cultivation to irrigated lowland rice, and we report independent observations from other agroecosystems in which soil remains anaerobic or partly anaerobic during the year and for which soil data suggest the covalent binding of soil N by lignin residues.…”

Evaluation of structural chemistry and isotopic signatures of refractory soil organic carbon fraction isolated by wet oxidation methods

“…These N compounds could be directly bonded to aromatic rings in lignin residues. Although aromatic binding of N has been demonstrated under laboratory conditions, the field detection attempts using NMR techniques were unsuccessful until when Schmidt-Rohr et al (2004) developed saturation-pulse-induced dipolar exchange with recoupling (SPIDER), an NMR technique involving MAS for selective detection of C that are bonded to N. With this advanced technique, significant amounts of N-bonded aromatic C around 134 ppm (N-C arom ) and N-bonded aliphatic C (N-CH) around 56 ppm were detected (Schmidt-Rohr et al 2004;Olk et al 2006). From the observed signal removal at 56 ppm and signal intensity reduction at 110-160 ppm upon NaOCl oxidation (Figs.…”