Land management effects on nitrogen and carbon cycling in an Ultisol

Abstract

Soil carbon (C) content in agro‐ecosystems is important in a global context because of the potential for soil to act as a sink for atmospheric CO₂. However, soil C storage in agro‐ecosystems can be sensitive to land management practices. The objective of this study was to examine the impact of land management systems on C and nitrogen (N) cycling in an Ultisol in Alabama. Soil samples (0–10, 10–20, and 20–30 cm depths) were collected from a Marvyn sandy loam soil (fine‐loamy, siliceous, thermic Typic Hapludults) under five different farm scale management systems for at least 5 years. The five systems were cotton (Gossypium hirsutum L.) production managed with 1) conventional tillage only, 2) conventional tillage with a grazed winter cover crop (wheat, Triticum aestivum L.), 3) conservation tillage with a winter cover crop grown for cover only with strip tillage; or taken out of cotton production with either 4) long‐term fallow (mowed), or 5) Conservation Reserve Program with loblolly pine (Pinus taeda L.) (CRP‐pine). Total N, total organic C (TOC), total P, and soil C:N ratios were determined. Potential C mineralization, N mineralization, C turnover and C:N mineralization ratios were determined on samples during a 30‐day laboratory incubation study. The fallow system had significantly higher TOC concentration (7.7 g kg^‐1 C) while the CRP‐pine system had lower TOC concentration (3.1 g kg^‐1 C) compared with the farmed management systems (=4.7 g kg^‐1 C). The fallow system had a significantly lower C turnover at all three soil depths compared with the other management systems. At the 0–10 cm depth, the highest C:N mineralization ratio levels were observed in management systems receiving the most tillage. Our results indicate that for Ultisols in the Southeast the use of surface tillage in land management systems is a controlling factor which may limit soil C sequestration.     

Continuous flow isotope ratio mass spectrometry of carbon dioxide trapped as strontium carbonate

Abstract

The isotopic signal provided by differential discrimination against atmospheric carbon dioxide (¹³CO₂) by C₃ and C₄ plant photosynthetic pathways is being widely used to study the processes of carbon (C) fixation, soil organic matter formation, and mineralization in nature. These studies have been facilitated by the availability of automated C and nitrogen (N) combustion analyzers (ANCA) combined with continuous flow isotope ratio mass spectrometers (CFIRMS). Analysis of ¹³CO₂ in these instruments requires consistent sample mass for best precision, a requirement that is easily satisfied for soil and tissue samples by adjusting sample weight. Consistent CO₂ sample size is much more difficult to achieve using gas handling systems for samples of headspace gases when CO₂ concentrations vary widely. Long storage of gaseous samples also is difficult. Extended respiration studies are most easily conducted by trapping CO₂ in alkali and conversion to an insoluble carbonate. Thermal decomposition of the carbonate in an on‐line ANCA allows consistent and optimal CO₂ sample mass to be obtained. The use of precipitated carbonates also facilitates storage of samples and enables full automation of sample analysis using an ANCA interfaced to a CFIRMS. Calcium (Ca), strontium (Sr), and barium (Ba) carbonates were tested. Strontium carbonate (SrCO₃) with the addition of vanadium pentoxide (V₂O₅) as a combustion catalyst was found most suitable.     

Carbon‐mineralization kinetics in an organically managed Cambic Arenosol amended with organic fertilizers

The dynamics of C mineralization in an organically managed Cambic Arenosol amended with hen manure, a stabilized compost (compost), and three different combinations of both fertilizers (varying from a 1:100 to a 1:10 ratio) were studied during an incubation experiment to estimate the potential of such combinations to preserve/restore soil C content relative to single applications. A strong increase of the CO₂‐C emissions relative to the unamended soil (control) was observed after soil application of all five organic‐fertilizer treatments. A significantly higher amount of applied C was lost in hen‐manure treatment (648 mg CO₂‐C [g C applied]^–1) when compared to compost (159 mg CO₂‐C [g C applied]^–1) or to the three combined treatments (176–195 mg CO₂‐C [g C applied]^–1). The first‐order exponential model and the double exponential model were used to fit the C‐mineralization data in the treatments considered. Results showed that mixing “small” amounts of hen manure with compost did not affect the total amount of potentially mineralizable C, but significantly increased the mineralization rate constant. Clearly, combinations of both fertilizers promoted an initial faster mineralization of the organic matter, and consequently a faster release of nutrients, without affecting the total amount of C sequestered in soil.     

Determination of the fate of 13C labelled maize and wheat exudates in an agricultural soil during a short-term incubation

A broader knowledge of the contribution of carbon (C) released by plant roots (exudates) to soil is a prerequisite for optimizing the management of organic matter in arable soils. This is the first study to show the contribution of constantly applied ¹³C‐labelled maize and wheat exudates to water extractable organic carbon (WEOC), microbial biomass‐C (MB‐C), and CO₂‐C evolution during a 25‐day incubation of agricultural soil material. The CO₂‐C evolution and respective δ¹³C values were measured daily. The WEOC and MB‐C contents were determined weekly and a newly developed method for determining δ¹³C values in soil extracts was applied. Around 36% of exudate‐C of both plants was recovered after the incubation, in the order WEOC < MB‐C < CO₂‐C for maize and MB‐C < WEOC < CO₂‐C for wheat. Around 64% of added exudate‐C was not retrieved with the methods used here. Our results suggest that great amounts of exudates became stabilized in non‐water extractable organic fractions. The amounts of MB‐C stayed relatively constant over time despite a continuous exudate‐C supply, which is the prerequisite for a growing microbial population. A lack of mineral nutrients might have limited microbial growth. The CO₂‐C mineralization rate declined during the incubation and this was probably caused by a shift in the microbial community structure. Consequently, incoming WEOC was left in the soil solution leading to rising WEOC amounts over time. In the exudate‐treated soil additional amounts of soil‐derived WEOC (up to 110 μg g⁻¹) and MB‐C (up to 60 μg g⁻¹) relative to the control were determined. We suggest therefore that positive priming effects (i.e. accelerated turnover of soil organic matter due to the addition of organic substrates) can be explained by exchange processes between charged, soluble C‐components and the soil matrix. As a result of this exchange, soil‐derived WEOC becomes available for mineralization.     

Nutrient Mineralization from Deoiled Neem Seed in a Savanna Soil from Nigeria

Abstract

The mineralization of nutrients from deoiled neem seed (neem seed cake), the residue left after oil extraction, was examined in a typical savanna soil with a view to determining its potential for fertility improvement. The neem seed cake (NSC) application rates were 0, 2.5, and 5.0 g kg^?1 soil (0, 5, and 10 tons ha^?1). The concentrations of ammonium‐nitrogen (NH₄‐N) and nitrate (NO₃)‐N mineralized from the neem‐amended soil were two to three times greater than the control. Similarly, exchangeable potassium (K), magnesium (Mg), and cation exchange capacity were significantly greater than the control. The neem‐amended soil maintained organic carbon (OC) at the pre‐incubation level, whereas OC in the control soil declined to significantly less than the pre‐incubation concentration. The electrolytic conductivity of the soil saturation extract with neem application was 8–10 times greater than the control soil. However, the NSC increased exchange acidity markedly and decreased the soil pH significantly. Thus, the benefits of NSC in increasing the concentrations of N, K, and Mg and maintaining OC of the soil must be weighed against the consequences of soil acidity, though it is unlikely that NSC can acidify the soil to the same extent under field conditions as it did in this closed‐system incubation study.     

Estimating Soil Carbon,Nitrogen, and Phosphorus Mineralization from Short‐Term Carbon Dioxide Respiration

The measurement of soil carbon dioxide (CO₂) respiration is a means to gauge biological soil fertility. Test methods for respiration employed in the laboratory vary somewhat, and to date the equipment and labor required have limited more widespread adoption of such methodologies. A new method to measure soil respiration was tested along with the traditional alkali trap and titration method. The new method involves the Solvita gel system, which was originally designed for CO₂ respiration from compost but has been applied in this research to soils with treatments of increasing dairy manure compost. The objectives of this research are to (1) examine the relationship between the CO₂ release after 1 day of incubation from soils amended with dairy manure compost that have been dried and rewetted as determined using the titration method and the Solvita gel system, and (2) compare water‐soluble organic nitrogen (N), as well as carbon (C), N, and phosphorus (P) mineralization after 28 days of incubation with 1‐day CO₂ release from the titration method and Solvita gel system. One‐day CO₂ from both titration and the Solvita gel system were highly correlated with cumulative 28‐day CO₂ as well as the basal rate from 7–28 days of incubation. Both methods were also highly correlated with 28‐day N and P mineralization as well as the initial water‐extractable organic N and C concentration.

The data suggest that the Solvita gel system for soil CO₂ analysis could be a simple and easily used method to quantify soil microbial activity and possibly provide an estimate of potential mineralizable N and P. Once standardized soil sampling and laboratory analysis protocols are established, the Solvita method could be easily adapted to commercial soil testing laboratories as an index of soil microbial activity.     

Relationship between rate of carbon dioxide evolution,microbial biomass carbon,and amount of dissolved organic carbon as affected by temperature and water content of a forest and an arable soil

Abstract

Using an Ochrept soil of a forest at climax stage or of an arable site at Kita‐Ibaraki, a city in central Japan, the rates of carbon dioxide (CO₂)‐carbon (C) evolution, the amounts of microbial biomass carbon (MBC) and the amounts of dissolved organic carbon (DOC) were measured in a laboratory with special reference to the incubation temperature and the soil water content. The rates of CO₂‐C evolution increased exponentially with increase in the incubation temperature in the range of 4–40°C. The temperature coefficients (Q₁₀) were 2.0 for the forest and 1.9 for the arable soil. The amounts of MBC were almost constant of 980 μg g^‐1 soil in the incubation temperature up to 25°C for the forest, and 340 μg g^‐1 soil in the incubation temperature up to 31 °C for the arable soil. The amounts of DOC in soil solutions were almost constant at 3.1 μg g^‐1 soil in the incubation temperature up to 25°C for the forest, and 3.8 μg g^‐1 soil in the incubation temperature up to 31°C for the arable soil. The rates of CO₂‐C evolution and the amounts of DOC increased with increase in soil water content (% of soil dry weight) up to 91% for the forest or up to 26% for the arable soil. However, the rates of CO₂‐C evolution and the amounts of DOC were almost constant within soil water content in the range of 91–160% or 26–53%, respectively. The amounts of MBC of the forest or arable soil were almost constant over a wide range of soil water content in the range of 41–220% or 8–73%, respectively. The rates of CO₂‐C evolution of both the forest and the arable soils were highly correlated with the amounts of DOC, but not with the amounts of MBC, under laboratory conditions in the case that the amounts of DOC were changed by various treatments. The regression equation,     

Potential underestimation of mineralization in 14carbon‐organochlorine biodegradation studies

Abstract

Two methods are described for capturing ¹⁴C‐carbon dioxide (¹⁴CO₂) from an alkaline clay soil in a study to determine the extent of mineralization of the chlorinated insecticide, endosulfan. In the first method, glass vials containing an aqueous alkaline solution [2 M sodium hydroxide (NaOH)] were incubated within soil microcosms and the ¹⁴CO₂ evolved and absorbed (into the alkaline solution), was determined directly. This simple and commonly used method, routinely reported in the literature, has been shown in the current study to be efficient and selective in absorbing ¹⁴CO₂, even at high concentrations of applied ¹⁴C‐substrate. A second method has been described for the recovery of indirectly evolved ¹⁴CO₂, i.e., dissolved ¹⁴CO₂ from the soil solution and ¹⁴CO₂ fixed in the soil as carbonate(s) and is designed for use as a check for the presence of these forms of ¹⁴CO₂. In the biodegradation trial in this study, very small amounts (<5%) of the originally applied ¹⁴C‐endosulfan I were shown to be converted to CO₂ over the trial period. It is recommended that during the design stage of establishing a biodégradation study, incubation conditions should allow for the capture all forms of ¹⁴CO₂ to avoid underestimating the areal extent of ¹⁴C‐substrate mineralization.     

Relative importance of substrate type and previous soil management in synthesis of microbial biomass and substrate mineralization

Our aim was to determine whether the soil microbial biomass, which has developed naturally over many years in a given ecosystem, is specially adapted to metabolize the plant‐derived substrate C of the ecosystem within which it developed or whether the nature of recently added substrate is the more important factor. To examine this, soils from three sites in close proximity (woodland, grassland and arable from the Broadbalk Experiment at Rothamsted Research, Harpenden, UK) were each amended with air‐dried wheat straw (Triticum aestivum), ryegrass leaves (Lolium perenne) or woodland leaf litter (mainly Quercus robur and Fagus sylvatica) in a fully replicated 3 × 3 factorial laboratory experiment. The initial mineralization rates (evolved CO₂‐C) were determined during the first 6.5 hours and again, together with the amount of microbial biomass synthesized (microbial biomass C), at 7, 14, 21, 30 and 49 days of incubation. The hourly rate of CO₂‐C production during the first 6.5 hours was slowest following leaf litter addition, while the added grass gave the fastest rates of CO₂‐C evolution both within and between soils. Ryegrass addition to the arable soil led to approximately four times more CO₂‐C being evolved than when it was added to the woodland soil, at an overall rate in the arable soils of 41 μg C g^?1 soil hour^?1. In each soil, the net amounts of CO₂‐C produced were in the order grass > straw > leaf litter. In each case, the amount produced by the added leaf litter was significantly less (P < 0.05) than either the added grass or straw. Overall, the trend was for much slower rates of mineralization of all substrates in the woodland soil than in either the arable or grassland soils. During 49 days of incubation in the woodland and grassland soils, the net total amounts of CO₂‐C evolved differed significantly (P < 0.01), with grass > straw > leaf litter, respectively. In the arable soil, the amounts of CO₂‐C evolved from added grass and straw were significantly larger (P < 0.01) than from the leaf litter treatment. Our findings indicated that the amounts of CO₂‐C evolved were not related to soil management or to the size of the original biomass but to the substrate type. The amount of biomass C synthesized was also in the order grass > straw > leaf litter, at all stages of incubation in the woodland and grassland soil. In the arable soil, the same effect was observed up to 14 days, and for the rest of the incubation the biomass C synthesized was in the order grass > straw > leaf litter. Up to three times more biomass C was synthesized from the added grass than from the other substrates in all soils throughout the incubation. The maximum biomass synthesis efficiency was obtained with grass (7% of added C). Overall, the woodland soil was most efficient at synthesizing biomass C and the arable soil the least. We conclude that substrate type was the overriding factor that determined the amount of new soil microbial biomass synthesized. Mineralization of substrate C by soil microorganisms was also influenced mainly by substrate type and less by soil management or size of original biomass.     

Weathered Fly Ash Does Not Affect Soil And Biosolid Carbon Mineralization

Fly ash and biosolid wastes can be mixed and applied to soil as a means of disposal. A significant decline in soil respiration following waste application indicates restricted activities of functional microbial populations. Weathering decreases salinity and neutralizes alkalinity in fly ash, but there is little information on the effects of unweathered fly ash and biosolid mixtures on soil carbon (C) mineralization. The objective of this study was to determine the effects of a weathered fly ash–limestone scrubber residue (LSR) mixed with an aerobically digested biosolid on soil respiration in a laboratory incubation study. Biosolids significantly increased carbon dioxide (CO₂) production (p < 0.05), but up to 6.75% (w/w) fly ash did not. Mean total C mineralization was 770 mg CO₂‐C kg^?1 soil in the control and 3,810 mg CO₂‐C kg^?1 soil in the 6.75% (w/w) biosolid treatment. Fly ash with neutral pH and low salinity appears unlikely to affect soil and biosolid C mineralization.     

Effect of methodological consideration on soil carbon parameter estimates obtained via the acid hydrolysis-incubation method

Many techniques such as the acid hydrolysis – incubation (AHI) method have been developed with the aim of elucidating the inherent complexity of soil organic carbon (SOC). While the utility of the AHI method has been demonstrated, there is no standardized protocol developed for conducting the long-term incubation component of the method. In the current study we evaluated the effects of chamber venting and mechanical headspace mixing on soil CO₂ flux rates and the resultant size and mean residence time of three operationally defined pools of SOC obtained via the AHI method. Continuous chamber venting resulted in an estimate of the readily mineralized carbon pool that was 2.3 times larger and turned over 2.9 times slower than the same pool estimated using periodically vented chambers. These differences were primarily attributed to the suppression of CO₂ flux in periodically vented chambers as a result of high internal CO₂ concentrations, and a concomitantly reduced diffusivity gradient. Prior to venting the periodically-vented chambers, CO₂ flux rates averaged 2.3 μg C (g soil)⁻¹ d⁻¹, while CO₂ flux rates following venting averaged 222.6 μg C (g soil)⁻¹ d⁻¹. We did not detect internal stratification of CO₂ suggesting that mechanical headspace mixing is unnecessary in incubation chambers ranging from 1 to 2 L. A standardized protocol is called for that isolates SOC fractions that are useful in hypothesis testing, while simultaneously seeking to minimize laboratory artifacts.     

The effect of young biochar on soil respiration

The low temperature pyrolysis of organic material produces biochar, a charcoal like substance. Biochar is being promoted as a soil amendment to enhance soil quality, it is also seen as a mechanism of long-term sequestration of carbon. Our experiments tested the hypothesis that biochar is inert in soil. However, we measured an increase in CO₂ production from soils after biochar amendment which increased with increasing rates of biochar. The ∂¹³C signature of the CO₂ evolved in the first several days of the incubation was the same as the ∂¹³C signature of the biochar, confirming that biochar contributed to the CO₂ flux. This effect diminished by day 6 of the incubation suggesting that most of the biochar C is slowly decomposing. Thus, aside from this short-term mineralization increasing soil C with young biochar may indeed be a long-term C storage mechanism.     

外加碳酸盐增加石灰性土壤密闭培养过程中CO2的释放量

The closed-jar incubation method is widely used to estimate the mineralization of soil organic C. There are two C pools (i.e., organic and inorganic C) in calcareous soil. To evaluate the effect of additional carbonates on CO2 emission from calcareous soil during closed-jar incubation, three incubation experiments were conducted by adding different types (CaCO3 and MgCO3 ) and amounts of carbonate to the soil. The addition of carbonates significantly increased CO2 emission from the soil; the increase ranged from 12.0% in the CaCO3 amended soil to 460% in the MgCO3 amended soil during a 100-d incubation. Cumulative CO2 production at the end of the incubation was three times greater in the MgCO3 amended soil compared to the CaCO3 amended one. The CO2 emission increased with the amount of CaCO3 added to the soil. In contrast, CO2 emission decreased as the amount of MgCO3 added to the soil increased. Our results confirmed that the closed-jar incubation method could lead to an overestimate of organic C mineralization in calcareous soils. Because of its effect on soil pH and the dissolution of carbonates, HgCl2 should not be used to sterilize calcareous soil if the experiment includes the measurement of soil CO2 production.     

Impact of cattle slurry acidification on carbon and nitrogen dynamics during storage and after soil incorporation

Acidification of animal slurry is recommended in order to reduce NH₃ emissions, but relatively little is known about the effect of such treatment on C and N dynamics during acidification, storage, and after soil application. A laboratory study was performed, and the CO₂ emissions from a high–dry matter slurry (HDM), a low–dry matter slurry (LDM), and the same respective acidified slurries (AHDM and ALDM) were followed during a storage period and after soil incorporation. The N‐mineralization and nitrification processes, as well as microbial‐biomass activity were also estimated in soil receiving both the acidified and nonacidified materials. We observed a strong CO₂ emission during the acidification process, and acidification led to a small increase in CO₂ emissions (≈ 11%) during storage of AHDM relative to HDM. No effect of LDM acidification on CO₂ emissions during storage was observed. About 30% of C released during storage of AHDM was inorganic C, and for ALDM the C release was exclusively inorganic. Soil application of AHDM and ALDM led to a decrease in soil respiration, nitrification, and microbial‐biomass‐C values, relative to soil application of HDM and LDM, respectively. Furthermore, it was shown that this effect was more pronounced in ALDM‐ than AHDM‐treated soil. Considering both steps (storage and soil application), acidification led to a significant decrease of C losses and lower C losses were observed from LDM slurries than from HDM slurries.     

Determination of the fate of regularly applied 13C‐labeled‐artificial‐exudates C in two agricultural soils

Exudates are part of the total rhizodeposition released by plant roots to soil and are considered as a substantial input of soil organic matter. Exact quantitative data concerning the contribution of exudates to soil C pools are still missing. This study was conducted to reveal effects of ¹³C‐labeled exudate (artificial mixture) which was regularly applied to upper soil material from two agricultural soils. The contribution of exudate C to water‐extractable organic C (WEOC), microbial biomass C (MBC), and CO₂‐C evolution was investigated during a 74 d incubation. The WEOC, MBC, and CO₂‐C concentrations and the respective δ¹³C values were determined regularly. In both soils, significant incorporation of artificial‐exudate‐derived C was observed in the WEOC and MBC pool and in CO₂‐C. Up to approx. 50% of the exudate‐C amounts added were recovered in the order WEOC << MBC < CO₂‐C in both soils at the end of the incubation. Newly built microbial biomass consisted mainly of exudates, which substituted soil‐derived C. Correspondingly, the CO₂‐C evolved from exudate‐treated soils relative to the controls was dominated by exudate C, showing a preferential mineralization of this substrate. Our results suggest that the remaining 50% of the exudate C added became stabilized in non‐water‐extractable organic fractions. This assumption was supported by the determination of the total organic C in the soils on the second‐last sampling towards the end of the incubation. In the exudate‐treated soils, significantly more soil‐derived C compared to the controls was found in the WEOC on almost all samplings and in the MBC on the first sampling. This material might have derived from exchange processes between the added exudate and the soil matrix. This study showed that easily available substrates can be stabilized in soil at least in the short term.     

Carbon sequestration in forest soils: effects of soil type, atmospheric CO2 enrichment, and N deposition

Soil contains the major part of carbon in terrestrial ecosystems, but the response of this carbon to enriching the atmosphere in CO₂ and to increased N deposition is not completely understood. We studied the effects of CO₂ concentrations at 370 and 570 μmol CO₂ mol^?1 air and increased N deposition (7 against 0.7 g N m^?2 year^?1) on the dynamics of soil organic C in two types of forest soil in model ecosystems with spruce and beech established in large open‐top chambers containing an acidic loam and a calcareous sand. The added CO₂ was depleted in ¹³C and thus the net input of new C into soil organic carbon and the mineralization of native C could be quantified. Soil type was the greatest determining factor in carbon dynamics. After 4 years, the net input of new C in the acidic loam (670 ± 30 g C m^?2) exceeded that in the calcareous sand (340 ± 40 g C m^?2) although the soil produced less biomass. The mineralization of native organic C accounted for 700 ± 90 g C m^?2 in the acidic loam and for 2800 ± 170 g C m^?2 in the calcareous sand. Unfavourable conditions for mineralization and a greater physico‐chemical protection of C by clay and oxides in the acidic loam are probably the main reasons for these differences. The organic C content of the acidic loam was 230 g C m^?2 more under the large than under the small N treatment. As suggested by a negligible impact of N inputs on the fraction of new C in the acidic loam, this increase resulted mainly from a suppressed mineralization of native C. In the calcareous sand, N deposition did not influence C concentrations. The impacts of CO₂ enrichment on C concentrations were small. In the uppermost 10 cm of the acidic loam, larger CO₂ concentrations increased C contents by 50–170 g C m^?2. Below 10 cm depth in the acidic loam and at all soil depths in the calcareous sand, CO₂ concentrations had no significant impact on soil C concentrations. Up to 40% of the ‘new’ carbon of the acidic loam was found in the coarse sand fraction, which accounted for only 7% of the total soil volume. This suggests that a large part of the CO₂‐derived ‘new’ C was incorporated into the labile and easily mineralizable pool in the soil.     

Nationally Coordinated Evaluation of Soil Nitrogen Mineralization Rate using a Standardized Aerobic Incubation Protocol

Abstract

Aerobic incubation methods have been widely used to assess soil nitrogen (N) mineralization, but standardized protocols are lacking. A single silt loam soil (Catlin silt loam; fine‐silty, mixed, superactive, mesic, Oxyaquic Arguidoll) was subjected to aerobic incubation at six USDA‐ARS locations using a standardized protocol. Incubations were conducted at multiple temperatures, which were combined based on degree days (DD). Soil water was maintained at 60% water‐filled pore space (WFPS; constant) or allowed to fluctuate between 60 and 30% WFPS (cycle). Soil subsamples were removed periodically and extracted in 2 M potassium chloride (KCl); nitrate (NO₃) and ammonium (NH₄) concentrations in extracts were determined colorimetrically. For each location, the rate of soil organic‐matter N (SOMN) mineralization was estimated by regressing soil inorganic N (N_i) concentration on DD, using a linear (zero‐order) model. When all data were included, the mineralization rate from four datasets was not statistically different, with a rate equivalent to 0.5 mg N kg^?1 soil day^?1. Soil incubated at two locations exhibited significantly higher SOMN mineralization rates. To assess whether this may have been due to pre‐incubation conditions, time‐zero data were excluded and regression analysis was conducted again. Using this data subset, SOMN mineralization from five (of six) datasets was not significantly different. Fluctuating soil water reduced N‐mineralization rate at two (of four) locations by an average of 50%; fluctuating soil water content also substantially increased variability. This composite dataset demonstrates that standardization of aerobic incubation methodology is possible.     

Measuring the CO2 production from maize‐straw‐amended soil columns— a comparison of four methods

A soil‐column experiment with maize‐straw application at different depths was carried out to investigate the accuracy of CO₂‐measurement systems in a greenhouse experiment with sandy and loamy soils. The classical approach of CO₂ absorption in NaOH solution was compared with three other methods using dynamic chambers. These methods were gas chromatography (GC), a portable infrared analyzer (IR), and a portable photo‐acoustic system (PAS). The cumulative CO₂ production over the 57‐day incubation period was significantly affected by the method and soil‐specifically by the treatments. The NaOH and GC method always formed a pair of lowest cumulative CO₂ production in all treatments with maize‐straw addition. In the treatments with bottom application of the maize straw, IR and PAS methods gave values at identical levels in both soils. In the treatments with top application of the maize straw, the IR method gave significantly highest values in the sandy soil and the PAS method in the loamy soil. The correlation coefficients between the cumulative CO₂ production of the three dynamic‐chamber methods (GC, IR, and PAS) and the static NaOH method were all significant, with r values between 0.90 and 0.93. The C balance can be used for testing the plausibility of CO₂‐production data. Roughly 102% (NaOH and GC) and 114% (IR and PAS) were recovered, including the CO₂‐production data in the C balance of the sandy soil. The respective data were 97% (NaOH and GC) and 104% (IR and PAS) for the loamy soil.     

Positive and negative carbon mineralization priming effects among a variety of biochar-amended soils

Pyrogenic carbon (biochar) amendment is increasingly discussed as a method to increase soil fertility while sequestering atmospheric carbon (C). However, both increased and decreased C mineralization has been observed following biochar additions to soils. In an effort to better understand the interaction of pyrogenic C and soil organic matter (OM), a range of Florida soils were incubated with a range of laboratory-produced biochars and CO₂ evolution was measured over more than one year. More C was released from biochar-amended than from non-amended soils and cumulative mineralized C generally increased with decreasing biomass combustion temperature and from hardwood to grass biochars, similar to the pattern of biochar lability previously determined from separate incubations of biochar alone.The interactive effects of biochar addition to soil on CO₂ evolution (priming) were evaluated by comparing the additive CO₂ release expected from separate incubations of soil and biochar with that actually measured from corresponding biochar and soil mixtures. Priming direction (positive or negative for C mineralization stimulation or suppression, respectively) and magnitude varied with soil and biochar type, ranging from −52 to 89% at the end of 1 year. In general, C mineralization was greater than expected (positive priming) for soils combined with biochars produced at low temperatures (250 and 400 °C) and from grasses, particularly during the early incubation stage (first 90 d) and in soils of lower organic C content. It contrast, C mineralization was generally less than expected (negative priming) for soils combined with biochars produced at high temperatures (525 and 650 °C) and from hard woods, particularly during the later incubation stage (250-500 d). Measurements of the stable isotopic signature of respired CO₂ indicated that, for grass biochars at least, it was predominantly pyrogenic C mineralization that was stimulated during early incubation and soil C mineralization that was suppressed during later incubation stages. It is hypothesized that the presence of soil OM stimulated the co-mineralization of the more labile components of biochar over the short term. The data strongly suggests, however, that over the long term, biochar-soil interaction will enhance soil C storage via the processes of OM sorption to biochar and physical protection.     

Carbon mineralization in an arid soil amended with organic wastes of varying degrees of stability

Abstract

This work investigated the kinetics of carbon (C) mineralization when different doses of organic materials with varying degrees of stability were added to an arid soil. Respiration assays showed that the incorporation of wastes led to a greater emission of carbon in the form of CO₂ and greater degree of microbial activity than those occurring in the control soil. Soils treated with fresh waste (municipal solid waste and sewage sludge) gave off more CO₂ than that treated with compost, with higher values being obtained at high than at low doses. Carbon dioxide emission was reduced with the length of time the organic materials remained in the soil. The data of cumulative CO₂ were fitted to the equation C=C₀(1‐e^‐Kt)+C₁. The parameters derived from this model were used as indices of organic matter decomposition, because the product of C₀ and K was more precise than either value separately. In all cases, an initial rapid phase of mineralization was clearly differentiated from a second slower phase.