Changes In Soil Organic Carbon Content Research Articles

Long‐term development of organic carbon contents in arable soil of North Rhine–Westphalia, Germany, 1979–2015

Europe currently faces a decline in soil organic carbon (SOC) content under arable cropping irrespective of improved management practices. In this study we used more than 32 000 SOC data from the entire North Rhine–Westphalia (NRW) area to elucidate the temporal development and current state of SOC content in arable soil from 1979 to 2015 and, in addition, to identify potential causal factors of changes in SOC content. The dataset comprised 7401 data points for 1979–2003 from FIS StoBo, the branched information system on soil contamination in NRW, and 24 930 data points for 2007–2015 from the Agricultural Investigation and Research Institute NRW (LUFA NRW); all data considered refer to soil sampled at 0–30 cm. We found that topsoil carbon contents in arable soil increased over the entire NRW area during the 1980s (from 1.87% in 1979 to 2.82% in 1987), and that this increase was followed by an exponential decrease from 1988 to 2015 (1.42% SOC in 2015) without a new equilibrium being reached. Livestock‐poor regions showed a more rapid gain and a more rapid loss of SOC contents than regions with large livestock densities. The data correlated with changes in grassland area, which was reduced in NRW by about 360 000 ha mainly between 1970 and 1990; that is, recently ploughed grassland probably contributed to increasing averages of SOC contents in arable land during this period, but not thereafter. We conclude that past changes in land use controlled the evolution in SOC from 1979 to 2015, and there is little opportunity for changed management practices to prevent ongoing loss of SOC from the topsoil.HighlightsThe SOC contents in arable soil in North Rhine–Westphalia (NRW) show temporal trend. The SOC contents increased during the 1980s, but decreased thereafter. Data from the entire NRW area indicate that new SOC equilibria have not yet been reached. Past changes in land use still control current SOC evolution.

Carbon input threshold for soil carbon budget optimization in eroding vineyards

Previous studies have documented that, relative to conventional tillage (CT), alternative soil management (reduced tillage, mulching, or cover crops) decreases soil erosion and increases soil organic matter (SOM) in vineyards. These previous studies, however, failed to consider the loss of soil organic carbon (SOC) with erosion that could occur with the adoption of agro-environmental measures (AEM) in a semiarid environment. Accordingly, the aims of this study were to determine whether changes in SOC content under AEM management are always positive and to develop a conceptual model for estimating the “SOC threshold”. The SOC threshold was defined as that level of SOC in an AEM-managed vineyard above which erosion will result in greater loss of C than occur in a comparable vineyard with CT management. SOC was analyzed at a 100 paired sites (vineyards with AEM management vs. CT). The results showed that in some cases the loss of C was higher with AEM than with CT. Overall, the results indicate that the SOC threshold may be a key parameter in determining the best AEM measures for vineyards that are on slopes and therefore vulnerable to erosion.

Differences in chemical composition of soil organic carbon resulting from long-term fertilization strategies.

Chemical composition of soil organic carbon (SOC) is central to soil fertility. We hypothesize that change in SOC content resulting from various long-term fertilization strategies accompanies the shift in SOC chemical structure. This study examined the effect of fertilization strategies along with the time of fertilizer application on the SOC composition by 13C nuclear magnetic resonance (NMR) spectroscopy. The soils (Aquic Inceptisol) subjected to seven fertilizer treatments were collected in 1989, 1999 and 2009, representing 0, 10 and 20 years of fertilization, respectively. The seven fertilizer treatments were (1–3) balanced fertilization with application of nitrogen (N), phosphorus (P) and potassium (K) including organic compost (OM), half organic compost plus half chemical fertilizer (1/2OM), and pure chemical NPK fertilizer (NPK); (4–6) unbalanced chemical fertilization without application of one of the major elements including NP fertilizer (NP), PK fertilizer (PK), and NK fertilizer (NK); and (7) an unamended control (CK). The SOC content in the balanced fertilization treatments were 2.3–52.6% and 9.4–64.6% higher than in the unbalanced fertilization/CK treatments in 1999 and 2009, respectively, indicating significant differences in SOC content with time of fertilizer application between the two treatment groups. There was a significantly greater proportion of O-alkyl C and a lower proportion of aromatic C in the balanced fertilization than in unbalanced fertilization/CK treatments in 1999, but not in 2009, because their proportions in the former treatments approached the latter in 2009. Principal component analysis further showed that the C functional groups from various fertilization strategies tended to become compositionally similar with time. The results suggest that a shift in SOC chemical composition may be firstly dominated by fertilization strategies, followed by fertilization duration.

Soil-specific effects of urea addition on mineralization of aromatic and proteinaceous components of humic-like substances in three agricultural soils

Humic substances (HS) are the most abundant components of soil organic carbon (SOC). Intensive use of inorganic N fertilizers in agricultural soils can alter stability and turnover of HS and hence C sequestration. Using specif- ically 14 C-labeled humic-like substances ( 14 C labeled in the aromatic or the proteinaceous component) as model HS, the effects of urea addition (0.1 and 0.2 mg N g �1 soil) on miner- alizationandtransformationofthearomaticandproteinaceous components of HS in three soil types (Anthrosols, Fluvisols, and Chernozems) were investigated. Our results showed that the urea addition had no effect on the mineralization of the aromatic and the proteinaceous components in the Anthrosol, whereas it significantly (P<0.05) inhibited (13.1-17.7 %) the mineralization of the two components in the Fluvisol. Urea addition (0.2 mg N g �1 soil) significantly (P<0.05) stimulated the mineralization of the aromatic (8.1 %) and the proteina- ceous (9.0 %) components in the Chernozem. The amounts of 14 Cindissolvedorganicmatter(DOM)inthethreetestedsoils were reduced after urea addition, probably because specific microbial species incorporating humic-like substances into DOM were inhibited by urea. The soil-specific humic-like substancesmineralizationafterureaadditionisconsistentwith the reported SOC change patterns of soil, indicating that the variable effects of urea addition on certain components of SOC could be partly responsible for changes in SOC contents induced by inorganic N fertilizers addition. Keyword 14 C.Decomposition .Transformation .N addition .Soilorganiccarbon

Changes in soil total organic carbon after an experimental fire in a cold temperate coniferous forest: A sequenced monitoring approach

Although numerous studies have demonstrated the effects of fire on the total amount of soil organic carbon (SOC), much remains unknown. In Autumn 2009, a well-controlled fire experiment was conducted in the boreal forest of Daxing'an Mountains, China. Over the next three years, repeated samplings were performed at settled points, taking the pre-fire state as the reference base (control). All soil samples were collected at a fixed sampling depth (0–10cm). Soil organic carbon content (SOCC) and soil organic carbon density (SOCD) were examined simultaneously. A paired-samples T-test was employed to ascertain any possible significant changes after the fire. The total amount of SOC in 0–10cm soil was dynamic after fire disturbance, and the direction and magnitude of the changes largely depended on the burning severity and the post-fire period. The spatial patterns of SOC changes and their temporal succession were closely associated with the primitive pattern of fire severity. For low- and medium-severity fires, no significant immediate change in SOCC or SOCD was detected. After one or two leaching seasons, a statistically significant increase (less than +5% on average for SOCC, less than +10% on average for SOCD) was observed in low- and medium-severity burning areas due to incorporation of fire-derived forest debris and semi-decomposed dead roots, but this minor increase was diminished or offset by the third year. High-severity fire caused significant immediate C loss in topsoil through direct combustion, while the subsequent changes varied: either rapidly and continually decreased until the third year (from approximately −10% to −60% for SOCC, and from −5% to −40% for SOCD) as a result of subsequent erosion, or increased dramatically (from +40% to more than +100% for SOCC) due to deposition of burnt OM and/or organic soil, depending on the convex or concave microtopography. The sensitivities of SOCC and SOCD to fire disturbance were inconsistent due to changes in soil bulk density. SOCD was relatively sensitive to the temporary increase after medium-severity fire, while SOCC was more sensitive to the prolonged decrease following high-severity burning. Factors affecting changes in SOC were more complicated than previously reported. In particular, the collapse, burning and erosion of topsoil all contributed to the downward-shift of actual sampling depth, which resulted in a “dilution effect” on SOCC and a “superaddition effect” on SOCD. This effect was an underlying reason for the sensitivity differentiation of the two indices and further complicated the characterization of changes in SOC.

Using the DNDC model to compare soil organic carbon dynamics under different crop rotation and fertilizer strategies

Soil organic carbon (SOC) plays a vital role in determining soil fertility, water holding capacity and susceptibility to land degradation. On the Chinese Loess Plateau, a large amount of crop residues is regularly removed; therefore, this agricultural area mainly depends on fertilizer inputs to maintain crop yields. This paper aims to use a computer simulation model (DeNitrification and DeComposition, or DNDC) to estimate the changes of SOC content and crop yield from 1998 to 2047 under different cropping systems, providing some strategies to maintain the SOC in balance and to increase crop yields. The results demonstrated that: (i) single manure application or combined with nitrogen fertilizer could significantly enhance the SOC content and crop yield on the sloped land, terraced field and flat land; and (ⅱ) in contrast to sloped land and terraced field, the SOC content and crop yield both continuously increased in flat fields, indicating that the flat field in this region is a good soil surface for carbon sequestration. These results emphasize that application of manure combined with nitrogen fertilizer would be a better management practice to achieve a goal of increasing soil carbon sequestration and food security.

Possible effect of soil organic carbon on its own turnover: A negative feedback

Soil carbon (C) sequestration plays an important role in mitigating climate change; therefore, it is essential to understand the mechanisms underlying soil organic carbon (SOC) turnover and dynamics. The humification coefficient of input organic materials (h) and the SOC decomposition rate (k) together control the SOC turnover, and they have been well known to be affected by factors including climatic factors, organic material qualities, soil characteristics, and anthropogenic activities. However, the SOC content changes over time, and thus far, whether the SOC content is an inherent factor has not been understood clearly. By using a new mathematical method, we found that SOC is possibly able to regulate its own turnover through a negative feedback: h decreases and k increases simultaneously with an increase in the SOC content. This feedback is caused by a series of chemical, biological, and physical mechanisms. We suggest that the response of the SOC pool to climate change is a function of the C input level to soil, temperature, and negative feedback of the SOC turnover; in addition, organic materials should be applied in priority to soils with low SOC content to enhance the soil C sequestration.

Soil organic carbon dynamics under different tillage systems in rotations with perennial pastures

Physical fractionation and 13C determinations are useful techniques for soil organic carbon (SOC) dynamics studies. Changes in SOC content, distribution and origin were assessed after 9.5-year crop-perennial (C3 species) rotation on a Uruguayan Mollisol under conventional tillage (CT) and no-tillage (NT). Soil samples were collected at depths of 0–6, 6–12 and 12–18cm in 1994 and 2003. Determinations were made of total SOC, particulate organic matter C (POM-C) and mineral-associated organic matter C (MAOM-C). In addition, 13C determinations were made on the total sample and the different particle size fractions. None of the studied variables were affected significantly by the tillage system. SOC levels in 2003 did not differ significantly from those of 1994 at any of the studied depths. However, changes were found in fraction distribution. Within 0–18cm of the soil surface, POM-C decreased by 63%, whereas MAOM-C did not vary significantly. After 9.5 years, only 14.5% of SOC within 0–18cm of the soil surface was young SOC. The largest proportion was incorporated within 0–6cm of the soil surface and in the coarsest physical fractions of organic matter. Only 17% of the estimated C input from crops for the study period was retained by the topsoil. The estimated half-life of SOC within the upper 18cm of soil was 28 years. Within this layer, the C half-life varied from less than 5 years for POM-C to more than 400 years for MAOM-C. These results suggest that agricultural rotation systems including perennial pastures are capable of maintaining SOC levels even under CT. However, C cycling and other ecosystem processes may be altered due to the significant loss of labile organic matter. The use of 13C analysis enabled the estimation of parameters relevant to the modeling of SOC dynamics.

Experiment design to achieve desired statistical power

The comparison of allocated treatments is a foundation for many studies in soil science. We often want to determine if a treatment applied to a soil has a significant effect compared with not applying that treatment (control). Often, a statistical test is performed to establish whether there is a true difference in the means of the treatment and control. The null hypothesis states that there is no difference in the means. There are two types of error that can be made when a statistical test is conducted. These are presented in Table 1. In soil science research, there is an overwhelming focus on controlling the Type I error (i.e., the value of a). When a comparison test yields a non-significant result, the usual reaction is to assume that there was no true affect of applying the treatment to the soil. What is often ignored is the risk of Type II error; that is, a true difference is not detected. In this case, the power was not sufficient to be able to detect the difference that existed. Power is defined as the probability of rejecting the null hypothesis. This probability depends on the magnitude of the true difference in the treatment means. A larger difference is easier to detect and will thus have higher power. A Type II error occurs when there is a failure to reject a false null hypothesis, and the risk of committing this error is higher when the number of treatment replicates is low (Barker Bausell and Li 2002). Soils are highly, spatially variable at multiple scales both laterally and vertically. There are also analytical errors which can impose variation in the laboratory. The number of treatment replicates must be sufficiently large so that the effect of the treatment can over-ride this inherent variability. Without sufficient replication to lower the variation, the null hypothesis will not be rejected. An example of a research area in soil science where Type II error is common is in the assessment of management effects on soil organic carbon content (SOC). Typically, long-term agroecosystem experiments (LTAEs) have been used to assess management effects on SOC (e.g., Janzen et al. 1998; VandenBygaart et al. 2011). The LTAEs were not usually initiated for the purposes of assessing SOC differences, but for agronomic purposes. They are typically randomized block designs with relatively small plot sizes, such that blocking effects can yield large variations in SOC between replicated plots (Ellert et al. 2007). Expected changes in SOC due to management practices can be small, and often require decades to detect (Janzen et al. 1998). This small effect size coupled with large betweenreplicated plot variation causes the power to decrease, and increases the chances of Type II error. Indeed, in some instances there can be significant effects of a certain management practice on SOC with low power to detect a difference in LTAEs. Yet, where there is not a difference detected between treatments, the researcher may have been inclined to accept that there was not a difference and thus be done with it. However, by reaching such a conclusion without conducting adequate statistical power analysis, there is a risk of not observing a difference that was actually present. Recently, there has been a concern that such errors can lead to erroneous interpretations of data, which can have implications on agricultural policy (e.g., VandenBygaart 2009). Kravchenko and Robertson (2011) highlight some examples of the recent literature where lack of statistical power can lead to erroneous interpretation of results. There are several approaches that soil scientists can take to ensure that Type II errors are reduced to acceptable levels. The first is to effectively control experimental variation amongst plots. This is achieved by effective blocking in the design of the experiment and by the use of covariates in the statistical analysis to control variation not removed by blocking. The second step is to determine the required number of replicate plots needed to ensure that power is acceptably high, after having reduced the experimental error variance to a minimum through effective blocking and covariate analysis. One may also exploit ‘‘hidden replication’’, through the use of factorial experiments, to increase power (e.g. Astatkie et al. 2006). Blocking is very widely used in field experiments. Commonly used blocked designs include both complete and incomplete blocks, Latin squares and split plots. Many excellent references exist to guide the choice of experimental design (for example, Kuehl 2000). Suffice to say that the goal in laying out blocks is always to minimize the variation amongst plots within a block and hence to maximize the variation amongst blocks, prior to applying the treatments. Of course, the plots within a

Changes in cropland topsoil organic carbon with different fertilizations under long-term agro-ecosystem experiments across mainland China

Topsoil soil organic carbon (SOC) data were collected from long-term Chinese agro-ecosystem experiments presented in 76 reports with measurements over 1977 and 2006. The data set comprised 481 observations (135 rice paddies and 346 dry croplands) of SOC under different fertilization schemes at 70 experimental sites (28 rice paddies and 42 dry croplands). The data set covered 16 dominant soil types found in croplands across 23 provinces of mainland China. The fertilization schemes were grouped into six categories: N (inorganic nitrogen fertilizer only), NP (compound inorganic nitrogen and phosphorus fertilizers), NPK (compound inorganic nitrogen, phosphorus and potassium fertilizers), O (organic fertilizers only), OF (combined inorganic/organic fertilization) and Others (other unbalanced fertilizations such as P only, K only, P plus K and N plus K). Relative change in SOC content was analyzed, and rice paddies and dry croplands soils were compared. There was an overall temporal increase in topsoil SOC content, and relative annual change (RAC, g kg(-1) yr(-1)) ranged -0.14-0.60 (0.13 on average) for dry cropland soils and -0.12-0.70 (0.19 on average) for rice paddies. SOC content increase was higher in rice paddies than in dry croplands. SOC increased across experimental sites, but was higher under organic fertilization and combined organic/inorganic fertilizations than chemical fertilizations. SOC increase was higher under balanced chemical fertilizations with compound N, P and K fertilizers than unbalanced fertilizations such as N only, N plus P, and N plus K. The effects of specific rational fertilizations on SOC increase persisted for 15 years in dry croplands and 20 years in rice paddies, although RAC values decreased generally as the experiment duration increased. Therefore, the extension of rational fertilization in China's croplands may offer a technical option to enhance C sequestration potential and to sustain long-term crop productivity.

A simplified modelling approach for quantifying tillage effects on soil carbon stocks

SummarySoil tillage has been shown to affect long‐term changes in soil organic carbon (SOC) content in a number of field experiments. This paper presents a simplified approach for including effects of tillage in models of soil C turnover in the tilled‐soil layer. We used an existing soil organic matter (SOM) model (CN‐SIM) with standard SOC data for a homogeneous tilled layer from four long‐term field experiments with conventionally tilled (CT) and no‐till (NT) treatments. The SOM model was tested on data from long‐term (&gt;10 years) field trials differing in climatic conditions, soil properties, residue management and crop rotations in Australia, Brazil, the USA and Switzerland. The C input for the treatments was estimated using data on crop rotation and residue management. The SOM model was applied for both CT and NT trials without recalibration, but incorporated a ‘tillage factor’ (TF) to scale all decomposition and maintenance parameters in the model. An initial value of TF = 0.57 (parameter uncertainty, PU = 0.15) for NT (with TF set to 1.0 for CT) was used on the basis of a previous study with observations of soil CO2 respiration. The simulated and observed changes in SOC were then compared using slopes of linear regressions of SOC changes over time. Results showed that the SOM model captured observed changes in SOC content from differences in rotations, N application and crop residue management for conventional tillage. On the basis of SOC change data a mean TF of 0.48 (standard deviation, SD = 0.12) was estimated for NT. The results indicate that (i) the estimated uncertainty of tillage effects on SOC turnover may be smaller than previously thought and (ii) simple scaling of SOM model parameters may be sufficient to capture the effects of soil tillage on SOM turnover in the tilled layer. Scenario analyses showed that the average extra C input needed to compensate for soil tillage was 762 (SD = 351) kg C ha−1 year−1. Climatic conditions (temperature and precipitation) also affected how much extra C was needed, with substantially larger inputs being required for wetter and warmer climates.

Regional Study of No‐Till Impacts on Near‐Surface Aggregate Properties that Influence Soil Erodibility

The extent to which tillage systems modify the near‐surface soil aggregate properties affecting soil's susceptibility to erosion by water and wind is not well understood. We hypothesized that an increase in soil organic carbon (SOC) content with conservation tillage systems, particularly no‐till (NT), may improve near‐surface soil aggregate properties that influence soil erodibility. This regional study assessed changes in aggregate resistance to raindrops, dry aggregate wettability, and dry aggregate stability as well as their relationships with changes in SOC content. Four long‐term (&gt;19 yr) tillage systems including moldboard plow (MP), conventional till (CT), reduced till (RT), and NT were chosen across the central Great Plains at Hays and Tribune, KS, Akron, CO, and Sidney, NE. The kinetic energy (KE) of raindrops required to disintegrate 4.75‐ to 8‐mm aggregates from NT soils equilibrated at −0.03 and −155 MPa matric potential was between two and seven times greater than that required for MP and CT soils in the 0‐ to 2‐cm depth in all soils. At the same depth, the water drop penetration time (WDPT) in aggregates from NT soils was four times greater at Akron and Hays and seven times greater at Sidney and Tribune compared with that in plowed soils. Aggregates from NT soils were more stable under rain and less wettable than those from plowed soils particularly in the surface 0 to 5 cm, but RT had lesser beneficial effects than NT management. The SOC content increased with NT over MP and CT and explained 35% of the variability across soils in aggregate wettability and 28% of the variability in resistance to raindrops in the 0‐ to 2‐cm depth. Aggregate wettability explained 47% of the variability across soils in KE of raindrops required for the disintegration of aggregates. No‐till management did not affect dry aggregate‐size distribution and stability except at Akron where mean weight diameter (MWD) in RT and NT was 50% lower than in MP management in the 0‐ to 2‐cm depth. Aggregates in MP and CT soils were either stronger or equally strong when dry but less stable when wet than in NT soils. Overall, NT farming enhanced near‐surface aggregate properties affecting erosion by water but had small or no effects on dry aggregate stability.

Soil organic carbon dynamics at the regional scale as influenced by land use history: a case study in forest soils from southern Belgium

AbstractLand use change (LUC) is known to have a large impact on soil organic carbon (SOC) stocks. However, at a regional scale, our ability to explain SOC dynamics is limited due to the variability generated by inconsistent initial conditions between sample points, poor spatial information on previous land use/land management history and scarce SOC inventories. This study combines the resampling in 2003–2006 of an extensive soil survey in 1950–1960 with exhaustive historical data on LUC (1868–2006) to explain observed changes in the SOC stocks of temperate forest soils in the Belgian Ardennes. Results from resampling showed a significant loss of SOC between the two surveys, associated with a decrease in variability. The mean carbon content decreased from 40.4 to 34.5 g C kg−1 (10.6 to 9.6 kg C m−2), with a mean rate of C change (ΔSOC) of −0.15 g C kg−1 year−1 (−0.023 kg C m−2 year−1). Soils with high SOC content tended to loose carbon while conversely soils with low SOC tended to gain carbon. Land use change history explained a significant part of past and current SOC stocks as well as ΔSOC during the last 50 years. We show that the use of spatially explicit historical data can help to quantitatively explain changes in SOC content at the regional scale.

Long-term soil organic carbon dynamics in a subhumid tropical climate: 13C data in mixed C3/C4 cropping and modeling with ROTHC

Scanty information on long-term soil organic carbon (SOC) dynamics hampers validation of SOC models in the tropics. We observed SOC content changes in a 16-year continuously cropped agroforestry experiment in Ibadan, south-western Nigeria. SOC levels declined in all treatments. The decline was most pronounced in the no-tree control treatments with continuous maize and cowpea cropping, where SOC levels dropped from the initial 15.4 to 7.3–8.0MgCha−1 in the 0–12 cm topsoil in 16 years. In the two continuously cropped alley cropping (AC) systems, one with Leucaena leucocephala and one with Senna siamea trees, SOC levels dropped to 10.7–13.2MgCha−1. Compared to the no-tree control treatments, an annual application of an additional 8.5Mgha−1 (dry matter) of plant residues, mainly tree prunings, led to an extra 3.5MgCha−1 (∼0.2% C) in the 0–12 cm top soil after 11 years, and 4.1MgCha−1 after 16 years. The addition of NPK fertilizer had little effect on the quantities of above-ground plant residues returned to the soil, and there was no evidence that the fertilizer affected the rate of SOC decomposition. The fact that both C3 and C4 plants returned organic matter to the soil in all cropping systems, but in contrasting proportions, led to clear contrasts in the 13C abundance in the SOC. This 13C information, together with the measured SOC contents, was used to test the ROTHC model. Decomposition was very fast, illustrated by the fact that we had to double all decomposition rate constants in the model in order to simulate the measured contrasts in SOC contents and δ13C between the AC treatments and the no-tree controls. We hypothesized (1) that the pruning materials from the legume trees and/or the extra rhizodeposition from the tree roots in the AC treatments accelerated the decomposition of the SOC present at the start of the experiment (true C-priming), and/or (2) that the physical protection of microbial biomass and metabolites by the clay fraction on this site, having a sandy top soil in which clay minerals are mainly of the 1:1 type, is lower than assumed by the model.

Quantifying laboratory and field variability to assess potential for carbon sequestration

Accurate measurements of soil organic carbon (SOC) levels are essential to assess changes in C sequestration rates. To this end we conducted studies to evaluate laboratory variability in SOC concentration measured at USDA-ARS laboratories in Akron, CO, Cheyenne, WY, and Lincoln, NE. At the Akron laboratory we also evaluated field spatial variability within common cropping treatments in order to assess the potential to quantify significant changes in SOC content associated with rotations of varying cropping intensities. Our data showed very low coefficients of variation for SOC values from each of the three laboratories, and the same average SOC values for soils from each treatment. For mitigating spatial variability, the data showed that a 10-ha field required 10 cores, the 0.2-ha field, 2 cores, and the 0.02-ha field, one core, in order to achieve a difference of ≤10% from the mean 95% of the time. With respect to cropping intensity, all rotations with fallow contained statistically the same SOC levels, with the continuous cropping treatment [winter wheat-(Triticum aestivum L.)–corn-(Zea mays L.)–proso millet (Panicum miliaceum L.) showing higher levels of SOC than the conventional-till winter wheat summer fallow and the wheat–sunflower (Helianthus annuus L.)–fallow. Data indicate that net changes in SOC content over time as a result of management will be very difficult to assess, and will require a sufficient minimum elapsed time, as well as great attention to sampling protocol.

Mechanisms of aggregate stabilization in Ultisols from subtropical China

The quantification and interpretation of aggregate stability depend on internal soil properties and external factors such as measurement method and aggregate size. The objectives of this study were to: (i) determine the aggregate stability in Ultisols from subtropical China applying the Le Bisssonais Method; (ii) determine the effect of initial aggregate size on its stability, and (iii) interpret mechanisms of aggregate stabilization in the soils. Three aggregate-size ranges (5–3, 3–2 and 2–1 mm) were obtained by dry sieving. After the wetting treatments, the dominant fraction of fragments for each soil was 2–1 mm or 0.63–0.2 mm. The mechanisms of aggregate breakdown was in the order, slaking>mechanical breakdown>micro-cracking. They differed with soil type and composition. The normalized mean weight diameter (NMWD) of the aggregates after fast wetting and wet stirring were more correlated with soil properties, such as degree of micro-aggregation (DOA), cation exchange capacity (CEC), K 2O, Fe 2O 3 or Al 2O 3 rather than clay and soil organic carbon (SOC) content. The binding force by soil organic matter was smaller than the force caused by entrapped air or the force of combination of mechanical stress by stirring and differential swelling of minerals. The smaller the aggregate, the larger was the aggregate stability according to NMWD. The rankings of the soils differed with the soil aggregate sizes and the wetting treatments. Sandy loams from sandstone (Sc and Sw) were the weakest soils while the purple mudstone (Pp) was the strongest. All the cultivated soils decreased in aggregate stability compared with the comparable uncultivated soils or parent materials irrespective of the cultivation time and the changes in SOC content after cultivation.

Effects of cropping systems on nitrogen, phosphorus and potassium forms and soil organic carbon in a Gray Luvisol

The effects of up to 23 years of agricultural cropping of a boreal forest soil on soil organic carbon (SOC) and N, P, and K pools were studied. The cropping systems studied were: (a) continuous barley, (b) continuous forage bromegrass, (c) continuous forage legume, and (d) barley/grass-legume forage rotation. Continuous bromegrass increased while other cropping systems decreased SOC in the surface soil. Kjeldahl N in soil approximately followed the trend in SOC. The net gain in N under continuous grass was attributed mostly to nonsymbiotic N fixation. Changes in SOC content appeared to be also influenced by cropping and tillage frequencies. Changes in fixed (intercalary) ammonium were small. There was no measurable change in total P, in part, because input was only slightly higher than crop offtake. Organic P increased under continuous bromegrass, and tended to decrease under continuous legume. The C/N and C/P ratios of soil organic matter decreased slightly with cropping. Exchangeable K (Kex) was decreased by cropping systems containing a legume crop to a greater extent than those without a legume crop. Most of the decrease occurred in the 0–15 cm depth. Nitric acid extractable K was not affected by cropping. Since net loss of Kex to 30 cm depth was substantially less than crop offtake, it is suggested that subsoil K reserves and matrix K were supplying a major portion of the crops' K requirement. It is concluded that the effects of cropping systems on SOC, N, P and K are influenced by crop type, and cropping and tillage frequencies.