Daniel C. Olk

ACCUMULATION OF LIGNIN RESIDUES IN ORGANIC MATTER FRACTIONS OF LOWLAND RICE SOILS: A PYROLYSIS-GC-MS STUDY

In tropical Asia, multiple annual cropping of lowland rice and the associated submerged soil conditions have been linked to long-term changes in nitrogen (N) cycling and the chemical nature of soil organic matter. To identify changes in organic matter properties, two humic acid fractions and whole soil samples were obtained from field treatments of lowland rice that varied in cropping intensity, fertilizer management, and location. These samples were methylated and analyzed by pyrolysis-gas chromatography-mass spectrometry. With compounds expressed in relative abundance, whole soil was enriched in nonmethoxybenzene compounds and heterocyclic N compounds compared with extracted humic acids. The young mobile humic acid (MHA) fraction had a wide diversity of methoxybenzenes that are associated with lignin residues. The more recalcitrant calcium humate (CaHA) fraction had characteristics intermediate between whole soil and MHA. Under intensified cropping and increased soil submergence, lignin residues increased in relative abundance in all three fractions. Heterocyclic N compounds decreased with intensified cropping, consistent with previous analysis by 15N nuclear magnetic resonance spectroscopy. Their parent compounds may be primarily naturally occurring heterocyclic N compounds. For whole soil, and especially the MHA, submergence effects were accentuated in treatments with high fertilizer rates, trends that may be related to the balance between input rates and degradation rates of crop residues. The ratio of myristic acid: stearic acid varied with soil submergence, fertilizer rate, and type of fraction in patterns following the abundance of methoxybenzenes. In general, responses of the MHA and CaHA to field treatments were representative of whole soil.

Maximum soil organic carbon storage in Midwest U.S. cropping systems when crops are optimally nitrogen-fertilized

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.

Enzymatically and ultraviolet-labile phosphorus in humic acid fractions from rice soils.

Humic acid (HA) is an important soil component that can improve nutrient availability and impact other important chemical, biological, and physical properties of soils. We investigated the lability of phosphorus (P) in the mobile HA and calcium humate fractions of four rice soils as measured by orthophosphate-releasing enzymatic hydrolysis and UV irradiation. Enzymatic hydrolysis releases hydrolyzable organic P, and UV irradiation abiotically releases P by breaking down phosphate-HA complexes. Less than 25% of the P in these fractions was detected in the soluble orthophosphate form (detectable soluble Pi). Enzymatic incubation increased detectable soluble Pi to 60% of total P. Treatment by UV irradiation alone released an additional 5% to 20% of humic-bound P compared with the untreated fractions. However, treatment by both UV irradiation and enzymatic hydrolysis released 0% to 14% of humic-bound P more than the amount of organic P that was enzymatically released from nonirradiated samples. The smaller amounts of P that were released from some humic fractions by both UV irradiation and enzymatic hydrolysis than by UV irradiation alone indicated some overlap between UV-degradable organic P and enzymatically labile organic P. Generally, about one half to two thirds of humic P in these rice soils was labile. These data demonstrated that either assumption that no P in humic fractions was labile P or all P in humic fractions was labile is not true. The lability of humic-bound P should be experimentally evaluated as reported in this work.

Soil Amino Compound and Carbohydrate Contents Influenced by Organic Amendments

Amino compounds (i.e. amino acids and sugars), and carbohydrates are labile organic components and contribute to the improvement of soil fertility and quality. Animal manure and other organic soil amendments are rich in both amino compounds and carbohydrates, hence organic soil amendments might affect soil processes through these labile components. This chapter first reviews the advances in research on soil concentrations of amino compounds and carbohydrates as impacted by animal manure and other organic amendments. The published papers are mainly on the amino compounds and carbohydrate changes in long-term field trials and laboratory or greenhouse incubations, tracking the fate of amendment-derived amino compounds and carbohydrates in soils. To investigate the amino compound and carbohydrate changes in the short-term (16 weeks), we present a greenhouse pot study (4–16 weeks) which demonstrated that poultry manure increased extractable amino compounds when applied to soil at a high rate, and depressed them at a low application rate. The increase at the high rate was further amplified in rhizospheric soils. Whereas microbial activities promoted carbon (C) and nitrogen (N) mineralization, the presence of ryegrass resulted in lower concentrations of amino compounds and carbohydrates. Finally, we recommend that more types of organic amendments be evaluated for their impacts on soil amino compound and carbohydrate levels, and the relationship in changes between the two types of compounds. Such cumulative knowledge would provide a basis for establishing the specific contributions of amino compounds and carbohydrates to soil N and C dynamics over the course of agricultural seasons and beyond.

Humic products in agriculture: potential benefits and research challenges—a review

Humic products have been used in cropland agriculture for several decades, but lack of widespread credibility has restricted their use to small proportions of farmers. To improve the credibility of humic products, we identify four knowledge gaps and propose pathways of future action to close these gaps. First, while the capacity of humic products to improve plant growth has been proven in greenhouse and growth chambers, more such work is needed in field conditions, especially to determine the modifying effects on humic product efficacy of environmental and management factors, including crop type, annual weather patterns, soil type, and fertility management. Many of the published field studies fail to address any of these factors. Second, full acceptance of humic products by the research community may first require a mechanistic explanation for plant responses to humic products. Some research groups are exploring plant-based mechanisms, but almost entirely in controlled conditions, not in field conditions. Industry often attributes yield responses to enhancement of soil nutrient availability without citing adequate evidence. Microbial-based explanations are also possible. Third, consumer trust in available humic products would be strengthened through industry-wide measures for quality control of humic product production and sale, including standard procedures for measuring their humic and fulvic acid contents and rapid bio-assays for distinguishing effective products from inert frauds. Finally, humic products are widely presumed to promote root growth, which offers the potential to increase soil C inputs and thereby improve soil health. Yet virtually, no such evidence has been presented, in part due to the absence of long-term field trials. Humic product companies in North America have organized a trade association to promote a more knowledge-based industry. To acquire a database that will support these objectives, we propose establishment of a global network of field sites that would measure crop responses to humic products across ranges of humic products, crop types, soil types, and climates. Plant and soil samples would be analyzed by cooperating specialists in advanced laboratories to identify mechanistic processes and benefits to both plant production and soil health. We believe the industry will indeed become more knowledge-based and the credibility of humic products will improve as (i) we learn more about their field efficacy across ranges of field conditions for improving crop yield and soil health, (ii) we gain further insights into possible mechanistic explanations, and (iii) the consumer gains the ability to discern genuine products from fraudulent materials.

Evidence from nuclear magnetic resonance spectroscopy of the processes of soil organic carbon accumulation under long-term fertilizer management

Summary Long-term application of fertilizers has a considerable effect on the accumulation of soil organic carbon (SOC), but the underlying processes remain unclear. We have examined the effects in the soil of two long-term (> 20 years) field experiments of a double-cropped maize (Zea mays L.)–wheat (Triticum aestivum L.) rotation on a Calcaric Fluvisol and paddy rice (Oryza sativa L) on a Hydragric Anthrosol. The chemical structures of the SOC were characterized with multiple cross-polarization magic-angle spinning 13C nuclear magnetic resonance spectroscopy. The treatments included organic fertilizer (OF) only, combined NPK (nitrogen, phosphorus and potassium) fertilizer with OF (NPKOF), mineral fertilizer pairings of NPK, NP and NK, and an unamended control. The continuous fertilizer treatments did not change the chemical composition of the Anthrosol SOC greatly except that the NPKOF treatment slightly enriched aromatic C and depleted O–alkyl C compared with the Control. All the treatments on the Anthrosol soil resulted in SOC accumulation, but to different extents. Principal component analysis of the Anthrosol and Fluvisol showed different relationships between the soil properties and functional groups. The Fluvisol SOC decreased in the order balanced fertilizer treatments (OF, NPKOF and NPK), NP, Control and NK treatments. The main structural changes of the Fluvisol SOC followed the same order with a decrease in the abundance of aromatic CO, aromatic C, anomeric C, O–alkyl C and OCH3/NCH, and an increase in COO/NCO and alkyl C, which led to an increase in alkyl–C/O–alkyl–C ratios and a decrease in aromaticity values. These results suggest that SOC accumulation with balanced fertilization on the Fluvisol can be attributed to less decomposition and more humification of the organic materials, whereas the smallest SOC content associated with P omission can be ascribed to more complete decomposition to gaseous end products. We consider that the chemical compositions of SOC may be altered by long-term fertilizer treatments, depending on the soil type. Highlights How does soil organic carbon (SOC) change in response to long-term fertilizer management? Improved understanding of SOC accumulation in Fluvisol and Anthrosol agricultural soils. Phosphorus omission in the Fluvisol retarded the accumulation and stimulated the decomposition of SOC. Fertilizer treatments affected chemical composition of SOC of the Fluvisol, but not the Anthrosol.

Distinct changes in composition of soil organic matter with length of cropping time in subsoils of a Phaeozem and Chernozem: Changes in organic carbon in subsoils with cropping time

Deeper soil horizons might provide an opportunity to enhance C sequestration because soil organic matter (SOM) at depth is assumed to be stable. However, it is unknown whether the stable composition of organic carbon in subsoils changes with the length of cropping time and the type of soil. The objectives of this study were to determine the effects on the chemical structures of SOM of cropping time after conversion from grassland to arable land under a Phaeozem and Chernozem in northeast China. Near‐quantitative multiple cross‐polarization (multiCP) ¹³C nuclear magnetic resonance (NMR) spectroscopy was applied, and 11 types of carbon (C) functional groups were identified. Principal component analysis of these functional groups showed that the chemical composition of SOM was differentiated by soil type and depth. The Phaeozem and Chernozem profiles differed mainly in their relative proportions of aromatic C–C and (CH₂)ₙ groups: the Phaeozem contained relatively more aromatic C–C, whereas the Chernozem contained relatively more (CH₂)ₙ groups. The fused‐ring aromatic C–C carbon was probably derived from char‐like organic matter generated by burning of plant litter or from SOM humification, whereas the (CH₂)ₙ groups were likely to be from plant‐ or microbially‐derived residues. The main differences between top‐ and sub‐soils were the occurrence of more protonated C in the topsoils and more non‐protonated C in the subsoils. With increasing length of cropping time, aromatic C–C and C–O groups and COO/N–C=O groups increased, but (CH₂)ₙ groups decreased in the Phaeozem subsoils and increased in the Chernozem subsoils. Our findings suggested that leaching and soil moisture might influence the origin, redistribution and transformation of the recalcitrant components of SOM in the soil profile, resulting in changes in SOM composition under different climates and soil types. HIGHLIGHTS: Characterization of chemical structures of soil organic matter (SOM) in soil profiles by solid‐state ¹³C NMR SOM composition varied between a Phaeozem and Chernozem and to a larger extent in the subsoils Fused‐ring aromatics and (CH₂)ₙ groups accumulated in subsoils of the Phaeozem and Chernozem, respectively SOM composition of subsoil changed probably with precipitation‐controlled translocation or transformation

Effects of crop rotation on properties of a Vietnam clay soil under rice-based cropping systems in small-scale farmers’ fields

In tropical deltas, intensive monoculture with three rice crops per year (RRR) has been the standard for decades. However, in recent years some farmers have started rice-based rotations with one or more upland crops per year. The trends for increased grain yields with this new system raises the question as to whether the introduction of upland crops affects properties of alluvial clay paddy soil. This was evaluated in the present study, which was performed at 40 paddy fields in the Vietnamese Mekong Delta under four different cropping systems (10 farms per system) on paddy rice soils: RRR; crop rotation with two rice crops and one upland crop per year (RUR); crop rotation with one rice and two upland crops per year (RUU); and upland crop (UUU). Soil samples were collected at depths of 0–10, 10–20 and 20–30cm. Most soil properties differed significantly between the RRR and the RUR, RUU or UUU. The RUR, RUU and UUU systems alleviated soil compaction, resulting in reduced penetration resistance and bulk density and increased total and macroporosity at 20–30cm depth. In addition, aggregate stability index and plant-available water capacity were higher for RUR, RUU and UUU compared with RRR at the 20–30cm depth. Average soil organic carbon (SOC) stocks ranged from 59.3tha–1 in UUU to 72.3tha–1 in RUR, with SOC stocks in RRR and RUU being intermediate (66.4 and 68.3tha–1) and not significantly different to that of the RUR system. Carbon hydrolysable by HCl (Chydrolysable) was 74–84% greater in the RUR, RUU and UUU than in RRR systems. In conclusion, rice–upland crop systems may alleviate soil degradation resulting from continuous rice monoculture.

Integrated Physical-Chemical Procedure for Soil Organic Carbon Fractionation and Characterization During Transition to Organic Farming

Two field experiments, in the south of Italy, were established in 2009 to study and characterize soil organic matter (SOM) during transition to organic farming. Experiments included a cereal/leguminous rotation fertilized with permitted amendments with three field replicates. A sequential fractionation procedure was used to separate different SOM fractions: light fraction (LF), two size classes of particulate organic matter (POM), mobile humic acid (MHA), and Ca-bound humic acid (CaHA). Isolated fractions were quantified and analyzed for their C and N content and carbohydrate and amino compound composition. The masses of the isolated fractions increased during 2-year course, with noticeable increases in LF and POM. Moreover, LF and POM were found more responsive than MHA to treatment and crop. The xylose/mannose ratio explained that MHA-carbohydrates were mainly of microbial origin, while LF- and POM-carbohydrates were of plant origin. Amino compounds constituted up to 30% of total soil N and were found to be more responsive to seasonal variation than to agronomic practices.