Below-ground Residues Research Articles

The Effect of Water Availability on the Carbon Content of Grain and Above- and Belowground Residues in Common and Einkorn Wheat.

The carbon (C) fixed by crops, which is exported with harvest and retained as postharvest residues in a field, is important for calculating the C balance. The aim of this study was to determine the effect of water availability on the C content in whole wheat plants. In a three-year field trial, the weights of grain, straw, chaff, stubble, and roots of two cultivars of winter wheat (Triticum aestivum L.) and one cultivar of einkorn wheat (Triticum monococcum L.) and their carbon contents were determined in water stress, irrigation, and rain-fed control treatments. The water availability, year, and cultivar had a significant influence on the C content in aboveground plant parts, but the effect of water on grain C was weak. The C content decreased with irrigation and increased with drought, but the differences were small (at most, 3.39% in chaff). On average, the C contents of grain, straw, chaff, and roots reached 45.0, 45.7, 42.6, and 34.9%, respectively. The amount of C exported with grain and left on the field in the form of postharvest residues depended on the weight of the total biomass and the ratio of grain to straw and residue. Whole plant C yield reached 8.99, 7.46, and 9.65 t ha-1 in rain-fed control, stressed, and irrigated treatments, respectively, and 8.91, 9.45, and 7.47 t ha-1 in Artix, Butterfly, and Rumona, respectively. Irrigation significantly increased the C content in grain and straw (but not in chaff, stubble, and roots) in comparison with water shortage conditions. On average, a grain yield of 1 t ha-1 corresponded to an average export of 0.447-0.454 t C ha-1 in the grain of all cultivars and inputs of 0.721, 0.832, and 2.207 t C ha-1 of residue to the soil in the form of straw and postharvest residue in the two cultivars of common wheat and one of einkorn. The results of the study provided reliable data for the calculation of the C balance of wheat under conditions of different water availability.

Challenges of accounting nitrous oxide emissions from agricultural crop residues.

Crop residues are important inputs of carbon (C) and nitrogen (N) to soils and thus directly and indirectly affect nitrous oxide (N2 O) emissions. As the current inventory methodology considers N inputs by crop residues as the sole determining factor for N2 O emissions, it fails to consider other underlying factors and processes. There is compelling evidence that emissions vary greatly between residues with different biochemical and physical characteristics, with the concentrations of mineralizable N and decomposable C in the residue biomass both enhancing the soil N2 O production potential. High concentrations of these components are associated with immature residues (e.g., cover crops, grass, legumes, and vegetables) as opposed to mature residues (e.g., straw). A more accurate estimation of the short-term (months) effects of the crop residues on N2 O could involve distinguishing mature and immature crop residues with distinctly different emission factors. The medium-term (years) and long-term (decades) effects relate to the effects of residue management on soil N fertility and soil physical and chemical properties, considering that these are affected by local climatic and soil conditions as well as land use and management. More targeted mitigation efforts for N2 O emissions, after addition of crop residues to the soil, are urgently needed and require an improved methodology for emission accounting. This work needs to be underpinned by research to (1) develop and validate N2 O emission factors for mature and immature crop residues, (2) assess emissions from belowground residues of terminated crops, (3) improve activity data on management of different residue types, in particular immature residues, and (4) evaluate long-term effects of residue addition on N2 O emissions.

Soil organic carbon dynamics in multipurpose cropping systems

Diversification of conventional food-based cropping system with multipurpose lignocellulosic crops could increase biodiversity, farmer’s market opportunities, feedstock for biorefineries, though can also imply soil organic carbon (SOC) stock changes, without competing with food production. Sunnhemp ( Crotalaria juncea L.), biomass sorghum ( Sorghum bicolor L.), kenaf ( Hibiscus cannabinus L.), and industrial hemp ( Cannabis sativa L.) are annual high yielding lignocellulosic crops with great potential for their multiple applications. The objective of the present study was to identify the SOC changes when the aforementioned crops are grown during the fallow periods of a conventional maize ( Zea mays L.)-wheat ( Triticum aestivum L.) rotation over five years. For this purpose, the SOC, carbon inputs (i.e. above and belowground residues), soil respiration , and cumulative CO 2 emissions were evaluated. The experiment was carried out in northern Italy (44°30′ N, 11°21′ E) from 2016 to 2021 in a randomized complete block design (n = 4) by adopting conventional field and harvest management practices (i.e. straw removal with hay machineries and alternating conventional with minimum tillage). The SOC stock increased by 7% from 2016 (56 Mg ha −1 ) to 2021 (60 Mg ha −1 ), regardless of the rotation system. Despite one of the highest CO 2 cumulative soil emissions (11 Mg ha −1 , two and three times higher than sunnhemp, kenaf, and industrial hemp, respectively) the rotation including biomass sorghum ended with higher SOC concentration (7.6 g kg −1 ) compared with industrial hemp (7 g kg −1 ) and kenaf (6.9 g kg −1 ), whereas fallow (7.2 g kg −1 ) and sunnhemp (7.1 g kg −1 ) showed intermediate values. The combination of high aboveground residues and high nitrogen fertilizations are deemed to act primarily on SOC level compared with belowground residues. In summary, the crop intensification did not deplete SOC, moreover biomass sorghum cropping system outperformed kenaf and industrial hemp through the decomposition into SOC of higher released residues. The rotation including sunnhemp coupled high SOC levels with low CO 2 emissions, raising interest in the light of its nitrogen fixing ability which can further help reducing the system’s carbon footprint. • Cropping systems with wheat and maize including lignocellulosic crops during the fallow period are presented. • Over five years, the SOC carbon stock increased by 7%, regardless of rotation. • The rotation resulting with the highest SOC concentration included biomass sorghum (7.6 g kg −1 ). • The rotation resulting with the lowest SOC concentration included industrial hemp (7 g kg −1 ), and kenaf (6.9 g kg −1 ). • Aboveground crop residues, reduced tillage and nitrogen fertilization are likely the main drivers influencing SOC.

Potential for the adoption of measures to reduce N2O emissions from crop residues in Denmark

Crop residues represent a climate change dilemma: they can promote carbon (C) sequestration, but they may also stimulate emissions of the powerful greenhouse gas nitrous oxide (N2O). Although there are crop residue management measures to reduce N2O emissions, N2O reductions achieved at national scale with these measures have been seldom studied, and how farmers' willingness to accept the measures constrains their potential remains largely unknown. Using Denmark as a case study, we combined a survey (completed by 592 farmers) and national data to assess the practical potential and obstacles for the successful implementation of management strategies to reduce N2O emissions from crop residues. Crop residue removal (particularly from vegetables and cover crops) and nitrification inhibitors were identified as effective in reducing N2O emissions from a biophysical perspective. If all aboveground crop residues from vegetables and cover crops were removed, N2O emissions could be reduced by 0.181 Gg N2ON, corresponding to 11% of the total N2O emissions from crop residues nationally. However, a low percentage of farmers would be willing to remove crop residues from the field, especially for vegetables and cover crops (25%), in connection to the possible short- to medium-term reduction in C sequestration. Similarly, use of nitrification inhibitors would reduce emissions by 0.247 Gg N2ON, corresponding to 15% of the total residue N2O emissions, and only 37% of all farmers would accept their use. Our results highlight that farmer’ preferences for the adoption of measures can constrain the use of the few available effective mitigation options. Better knowledge dissemination and advisory services are crucial to address this challenge; farmers may be motivated to remove aboveground crop residues by highlighting the proportionally more important contribution of belowground residues to C sequestration, and that aboveground residues may have commercial value (biorefining, biogas, biofuel), although these options need further development.

Peanut residue incorporation benefits crop yield, nitrogen yield, and water use efficiency of summer peanut – winter wheat systems

Summer peanut – winter wheat is considered a feasible alternative to the traditional summer maize – winter wheat on the North China Plain, and it has great potential to maintain crop yield and reduce environmental costs. However, the yield, water consumption, and nitrogen utilization of subsequent wheat responses to peanut residue incorporation are not well-documented. Thus, a two-year field experiment with peanut root, shoot, and total residue incorporations, as well as a control with no residue, was established for the summer peanut – winter wheat cropping system. Peanut residue incorporation had no significant yield benefits for subsequent wheat and subsequent peanut, compared with no peanut residue input. The wheat yield was positively correlated with previous peanut yield, due to complementary utilization of residual nitrogen and water resources. Therefore, residue incorporation increased the annual yield of the summer peanut – winter wheat system by 5–13%, decreased the annual evapotranspiration by 3–12%, and boosted annual water use efficiency by 8–26%. Yield increases caused by peanut residue incorporation were detected only from the perspective of multiple cropping systems rather than single crop seasons. This indicated that at least two crop seasons were needed to verify the effects of residue incorporation on the North China Plain. Remarkably, no significant differences in yield, nitrogen yield, or wheat equivalent yield were detected between root and total residue incorporations. This clearly indicates that shoot removal has a negligible effect on the annual crop yield and performance of the summer peanut – winter wheat system. In conclusion, the yield benefits of legume residue incorporation are mainly attributed to the belowground residue, and the benefits may be underestimated when only one crop season after residue retention was investigated.

Roots and other residues from leys with or without red clover: Quality and effects on N2O emission factors in a partly frozen soil following autumn ploughing

Revised IPCC guidelines assume that a constant share of N in decomposing crop residues is directly emitted as N2O (emission factor: EFN2O), and calculate the amount of nitrogen (N) in non-removable residues of temporary grasslands proportionally to the average annual herbage yield. However, EFN2O depends on the intrinsic quality of the residues and their interactions with environmental conditions. Only a few field studies on N2O emissions from grassland renewal are available, and none have simultaneously quantified the N amount and quality of non-removable residues (roots and stubble).To gain insight into the effect of non-removable residue quality on EFN2O, we studied the amount and quality of roots and stubble and their effect on EFN2O following the ploughing of three-year-old swards.The measured amount of N in non-removable residues was approximately 20, 25, and 31 kg N per 1 Mg average annual dry matter yield in grass, red clover-grass, and red clover, and 70–83% of it was below ground. However, the EFN2O of non-removable residues measured over 252 days was lower (0.24%, SE = 14% for grass and red clover-grass) than the IPCC default value (0.6%, CV: 50%) for wet regions, although within the uncertainty margin, and was significantly lower than the EFN2O of incorporated herbage, which was related to differences in EFCO2. We advocate for more specific studies that separate the effects of belowground and aboveground residues (AGR), considering the possibility of simplifying the accounting of N2O emissions from belowground residues while improving that of non-removable AGR from temporary grasslands and other green crops.We observed the accumulation of N2O in the frozen soil under snow, which was released during diurnal percolation of meltwater. N2O emissions from frozen soil accounted for 30% or more of the total emissions.

Carbon footprint and net carbon gain of major long-term cropping systems under no-tillage

Due to increased contribution from agriculture sector to total greenhouse gas emissions, there is need to study the ability of no-tilled diverse cropping systems including crop sequences and bio-covers to mitigate C equivalent emissions. Thus, C-footprint was calculated for a long-term experiment at the University of Tennessee's Research and Education Center in Milan with six-crop sequences: continuous cotton (Gossypium hirsutum L.), cotton-corn (Zea mays L.), continuous corn, corn-soybean (Glycine max L.), continuous soybean, and soybean-cotton interacted with four bio-covers: poultry litter, hairy vetch (Vicia villosa), winter wheat (Triticum aestivum), and fallow control with three replicates in a strip-plot design. During the experiment duration (2002–2017), field inputs (fertilizers, pesticides, and machinery used for planting, chemical applications, and harvesting) and outputs (crop yield, aboveground, and belowground residue) were assessed for each crop sequence/bio-cover combination to calculate total C equivalence of inputs and outputs, net C gain, C footprint per kg yield, sustainability index, and nitrous oxide emissions. For continuous corn, C-based input emissions were significantly higher by 0.28–0.62 Mg CO2 eq. ha−1 yr−1 than all other sequences, however, a greater net C gain (5.4 Mg C eq. ha−1 yr−1) was also observed due to increased crop yield, aboveground and belowground residues. Poultry litter application resulted in lower C-footprint (1.59–2.09 kg CO2 eq. kg−1 yield) than hairy vetch, wheat, and fallow under all crop sequences. Hairy vetch also lowered C-footprint per kg yield (∼2–14%) when compared with wheat under continuous systems of corn, soybean, and cotton, and cotton-corn rotation. Poultry litter application increased sustainability index (23–45) of all cropping sequences compared with other bio-covers. Hairy vetch improved sustainability index of corn including cropping sequences as compared with wheat and fallow. Inclusion of soybean and cotton with corn significantly decreased nitrous oxide emissions by 20–25%. The major factor contributing towards C-based input emissions was N fertilizer with 68% contribution to total emissions on average. It is concluded that application of poultry litter can reduce per yield C-footprint and enhance production system sustainability compared with hairy vetch, wheat, and fallow for monocultures or rotations of corn, soybean, cotton. Additionally, hairy vetch can outperform wheat in reducing the per yield C-footprint for continuous corn/soybean/cotton, and cotton-corn rotation. Especially for corn production systems, hairy vetch can enhance sustainability index compared with wheat and fallow. In order to increase per hectare net C gain, reduce per yield C-footprint and enhance sustainability index simultaneously, integration of continuous corn or corn-soybean/cotton rotation with bio-cover poultry litter or hairy vetch may perform better than the monocultures of soybean or cotton integrated with bio-cover wheat or fallow control in the Mid-south USA.

Towards integrated cover crop management: N, P and S release from aboveground and belowground residues

Ecological intensification is crucial for arable organic farmers, in order to partly close the yield gap to conventional yields. One strategy for this is improved management of cover crops, to increase availability of both nitrogen (N), phosphorus (P) and sulfur (S) for the subsequent main crop. However, the potential of cover crops to take up and release nutrients has primarily been determined for aboveground biomass and mostly for N. There have been fewer studies on belowground biomass, and these have focused on recoverable roots. Thus, unrecoverable residues from rhizodeposits and nodules, have been overlooked. The aim of this incubation study was therefore to assess the release of N, P and S from aboveground residues and from intact soil with belowground residues of different cover crop species at low temperature. Shoots and intact soil with roots from seven different cover crops were incubated for 80 days at a temperature of 5 °C.A comparison of the cover crop species revealed a wide variation in nutrient uptake, chemical composition and release of N, P and S, that also varied between shoot and intact soil with roots. Release of N from shoots showed a marked non-stoichiometric behavior, in contrast to the release of P and S, that correlated well with C:P and C:S ratios. N release (from Vicia villosa) and especially P mobilization (from legumes) in intact soil was much greater than the amounts determined in recoverable roots. This shows that unrecoverable pools such as rhizodeposits and fine roots contributed to nutrient release. Garden sorrel (Rumex acetosa) is a promising cover crop regarding uptake and mobilization of P, but also as a multifunctional cover crop for supplying N, K, S and Mg. Hairy vetch (Vicia villosa) is a promising cover crop for N, oilseed radish (Raphanus sativus) for S, P and K, and white lupin (Lupinus albus) for N, P and Mn supply.Based on our results, we propose the use of mixtures of cover crops with different N, P and S uptake and release having the potential to supply a more complete suite of nutrients to the subsequent crop.

Crop residues contribute minimally to spring-thaw nitrous oxide emissions under contrasting tillage and crop rotations

Crop residues are sources of carbon and nitrogen (N) after harvest, releasing inorganic N through mineralization or protecting soil N through immobilization. Inorganic N controls nitrous oxide (N2O) emissions, a potent greenhouse gas (GHG) from agriculture. Hence, crop residues are accounted for as N2O sources in national GHG inventories. For locations where post-harvest N2O emissions occurs due to freeze-thaw, it is not known if crop residues contribute to emissions, and if tillage or residue type impact this contribution. This is of concern since crop residue and freeze-thaw emission factors (EF) may be ‘double-counting’ N2O sources. We conducted an experiment over two non-growing seasons (NGS) in a long-term corn, soybean and winter wheat trial to (i) compare N2O emissions for different crop residues within simple or diverse crop rotations under no-tillage (NT) or conventional tillage (CT); (ii) determine the importance of above- and below-ground residue addition to spring-thaw N2O emissions as affected by rotation and tillage. A15N residue enrichment study was used to directly trace above- and below-ground residue 15N into 15N2O fluxes and derive EF. Higher N2O emissions were observed for CT than NT, regardless of rotation. Soybeans induced higher N2O emissions than corn residue and the same crop residue (e.g. corn or soybean) showed trends of higher N2O in the long-term diverse rotation. In all cases, crop residues contributed minimally to spring-thaw N2O emissions (<2%), meaning differences in N2O emissions were due to tillage and rotation effects on soil N availability, rather than by affecting crop residue N release for N2O production. The NGS 6-month EF for crop residues never surpassed 0.05%, a minimal fraction of the annual 0.6% recommended EF. Refinement in emission inventories for cold climates should focus on freeze-thaw substrate release for N2O production from other sources than crop residue.

Modeling Hairy Vetch and Cereal Rye Cover Crop Decomposition and Nitrogen Release

Empirical models could help us to understand the process of plant residue decomposition and nutrient release into the soil. The objective of this study was to determine an appropriate model to describe the decomposition of hairy vetch (Vicia villosa Roth) and cereal rye (Secale cereale L.) cover crop (CC) residue and nitrogen (N) release. Data pertaining to above and belowground CC residue mass loss and N release for up to 2633 cumulative decomposition degree days (112 d) after litterbag installation were obtained from two cropping system experiments, a 1-yr study conducted in 2015 and a 2-yr study during 2017 to 2018 in the humid subtropical environment of southern IL, USA. Six exponential and two hyperbolic models were fit to percent mass and N remaining data to find the one with minimum Akaike information criterion (AIC) and residual sum of squares. Modified three-parameter single exponential and two- or three-parameter hyperbolic models best met the assumed criteria of selection for above and belowground CC residue, respectively. Fitting a double exponential model to combined data for percent mass and N remaining identified two mass and N pools, a fast and a slow pool with different rate constants. A five-parameter double exponential with an asymptote met the preset criteria and passed all tests for normally distributed population, constant variance, and independence of residuals at α = 0.05 when fit to combined data of hairy vetch shoot mass and N remaining. However, a two-parameter hyperbolic and three-parameter asymptotic hyperbolic model provided the best fit to a combined data of cereal rye shoot mass and N remaining, respectively. Both hyperbolic decay models showed a good fit for belowground mass decomposition and N release for both CCs. Cereal rye had a poorer fit than hairy vetch for mass and N remaining of both above and belowground mass. The best-selected decay models can be used to estimate the decomposition and N release rates of hairy vetch and cereal rye above and belowground residue in a similar environment.

A side-by-side comparison of biological nitrogen fixation and yield of four legume crops

Evaluate potential N benefit from chickpea (Cicer arietinum L.), faba bean (Vicia faba L.), lentil (Lens culinaris L.) and field pea (Pisum sativum L.). This is the first phase of a 2-year cropping sequence study quantifying above-ground and below-ground residue contributions to N uptake by subsequent crops. The four legume crops were grown in field experiments. Biological N fixation (BNF) was quantified by 15N isotope dilution. Faba bean fixed the most N (76%) and had the highest seed yield but because it exported >80% of N fixed to seed it had the second lowest residue N (39 kg ha−1). Chickpea and lentil fixed comparable amounts of N (62%) but had low seed yields and hence higher residue N (56 kg ha−1 and 40 kg ha−1, respectively). Field pea fixed the lowest amount of N (50%) but had the second highest seed yield and consequently the lowest residue N (24 kg ha−1). High BNF does not reflect potential N benefit. Distribution of N to seed and residue were different among the legume crops, with chickpea and lentil being more sensitive than pea or faba bean to different climate conditions.

Carbon and nitrogen stock of Acrisols and Nitisols in South Bahia, Brazil

This study evaluated if the effect of secondary forest (SF) conversion into pasture (PAS) or rubber tree plantation (RT) and cacao + erythrina AFS (C + E) conversion into cacao + rubber tree AFS (C + RT) on the C and N storage in soils and aggregate-size fractions could be compensated by a high amount of organic residues deposited over the years by rubber tree plantation and agroforestry systems (above and belowground) and a high amount of belowground residues deposited by pasture. Soil samples were collected from six layers (0–10, 10–20, 20–40, 40–60, 60–80 and 80–100 cm) and separated by wet-sieving into three fraction-size classes (2000-250 μm; 250-53 μm and < 53 μm) in soils under RT, PAS and SF sites (Argisols) and C + RT and C + E AFSs (Nitosols). The C and N were determined by dry combustion. The average soil organic carbon (SOC) stock in Acrisols was 207 Mg ha−1 up to 100 cm depth, and 72 Mg ha−1 was stored at the first 30 cm depth; while the average SOC stock in Nitosols was 224 Mg ha−1 up to 100 cm, and 116 Mg ha−1 at 30 cm depth. As expected, C and N stock in soils and in the different soil aggregate-size fractions revealed that the dissimilarity between sites was strongly influenced by soil order. There was high dissimilarity between the sites in Nitosols and those in Yellow Argisols, and the dissimilarity between the land use systems only occurred in the Yellow Argisol. The conversion effect of cacao + erythrina AFS into cacao + rubber tree AFS was restricted to the surface soil and did not promote dissimilarity among the AFSs regarding the C and N storage in the whole soil and aggregate size-fractions. Adopting no tillage and tree-based systems (such as the rubber tree plantation) would compensate the negative conversion effect in the amount of organic matter and promote C and N storage mostly in the soil aggregate size-fractions. The effect of land-use conversions was more evident in the aggregates than in the whole soil, regardless of the soil order. The conversions in both soil orders induced macroaggregate turnover and increased the yield of new free microaggregates.

Building up the soil carbon pool via the cultivation of green manure crops in the Loess Plateau of China

The growth of summer legumes as green manures (GM) may influence the soil C pool, thereby affecting soil quality and productivity in a traditional cropping system. However, there is a lack of research on whether the inclusion of GM crops enhances the C economy under semiarid conditions. An ongoing field experiment was conducted on Cumulic Haplustoll in the northwest region of China to (i) quantify the biomass C release patterns of three GM legumes; (ii) evaluate the effect of the GM legumes on the soil organic carbon (SOC) stock (0–20 cm depth), and (iii) estimate the soil C dynamics for the GM systems under semiarid conditions in more detail. The results showed that the percentages of the remaining GM C in the soil after one year of decomposition averaged 26% and 33% for the above-ground and below-ground residues, respectively. Thus, the 5-yr growth of GM legumes continuously significantly improved the SOC and easily oxidized organic carbon (EOOC) concentrations, as well as the corresponding stocks (except for soybean) (P < 0.01), compared with the original soil at the 0–20 cm depth. We estimated the annual decomposition rate of SOC as 1%, and the C added by the crops that remained in the SOC as 23% each year using the one-compartment model. The mean turnover time of the SOC at equilibrium was estimated to be 22 years, indicating that the loess soil was not C saturated and still had the potential for C sequestration. In general, Huai bean performed better on biomass production, C accumulation, and soil C sequestration than mung bean and soybean under the dryland conditions during the 5-yr period of this study. Consequently, the cultivation of leguminous GM during the summer is a better option than bare fallow to improve the soil fertility and environmental sustainability not only in the Loess Plateau of China but also in other similar dryland regions.

Dry matter partitioning and residue N content for 11 major field crops in Canada adjusted for rooting depth and yield

To improve the estimates of C and N inputs to soil, we developed new estimates of partitioning between the harvested portion, aboveground residue, and belowground residue for 11 major crops based on depth-adjusted root/shoot ratios and grain yield-adjusted harvest indices. We updated the mean N concentration of each partition.

Dual-labelling (15N and 33P) provides insights into stoichiometry and release of nitrogen and phosphorus from in situ mature lupin and canola below-ground residues

Belowground (BG) residues may contribute significant amounts of N and P via nutrient cycling to following crops, particularly in low fertiliser input systems or where all the above-ground residues are removed. Reports of simultaneous measures of nitrogen (N) and phosphorus (P) release from mature crop residues, especially from intact root systems in situ, are rare. A single stem-feed of 15N followed by 33P was used to (i) estimate total amounts of N and P accumulated in situ BG by mature canola and lupin plants including the stoichiometry of N and P in recovered and non-recovered components of these BG residues, and ii) simultaneously trace and quantify the relative release of that N and P. One set of pots were destructively sampled at lupin and canola maturity to quantify total accumulation of BG P and BG N, including estimated N and P in unrecovered roots plus root-derived materials (RD N and RD P). Shoots were removed from a second set of pots into which wheat was sown after a 3 weeks fallow. Release of P and N from the decomposing in situ 33P/15N labelled lupin and canola BG residues was assessed as uptake in 5 weeks old wheat. Canola root had higher P and lower N concentrations than lupin. Canola total BG P was greater than lupin with a higher proportion as estimated RD P. Estimated RD N was similar in both species but lupin had more N in roots and so higher total BG N. C:P ratio of lupin roots was 708:1 and 188:1 for canola. Root C:N ratio was 39:1 for canola and 24:1 for lupin. N:P ratio for lupin roots was wider (29:1) than canola (5:1), but N:P ratio of RD fractions was similar (6:1 canola; 7:1 lupin). Proportion of BG P released and taken up by wheat was 21% after canola and 19% after lupin, and since total amount of BG P was much greater for canola the quantity of P released was double that after lupin. Proportion of lupin BG N (37%) released was similar to that for canola BG N (33%) although a larger amount of N was released from lupin given the larger BG N pool. The proportion of P and N released from in situ BG residues of mature canola and lupin and taken up by wheat in this short term study was broadly inversely related to C:P, C:N and N:P ratio of recovered roots but results suggest a likely influence also of N:P ratio of the unrecovered BG residues. Quantities of N and P released were a function of the estimated total amount of plant N and P accumulated in situ BG.

Residual plant nutrients in crop residues – an important resource

ABSTRACTWith increasing demand for sustainable production, the need to effectively utilise site-based resources increases. One such resource is the remaining crop residues, both above and below ground, after harvest. In order to assess the magnitude of this resource, this study determined the plant nutrient contribution of residues from 17 different crops in seven different regions of Slovakia over a nine-year period. The soil profile 0.0–0.3 m was taken into account at belowground residues sampling. The results showed that winter rape, mustard, corn maize, triticale, winter rye with straw and sunflower left the most residues, exceeding 8 tons of dry biomass per hectare. Root crops (sugar beet, potatoes) and peas left less than 3 tons of residual dry biomass per hectare. Using these data, the nutrient potential coefficient, i.e. the amount of nutrients left by production of one ton of the main product was calculated by polynomial regression. With these coefficients and the known yield of a given crop, it was possible to calculate how much nitrogen, phosphorus and potassium each crop leaves in the soil after harvest. It was found that the amount of nitrogen left ranged from 20 to 132 kg, phosphorus from 2 to 24 kg and potassium from 13 to 218 kg per hectare. This has to be taken into account when calculating the fertiliser requirement of the subsequent crop in order to achieve better resource utilisation, thereby reducing the risk of eutrophication and improving farm profits by reducing expenditure on fertiliser.

Soil Carbon Fractions in Response to Long‐Term Crop Rotations in the Loess Plateau of China

Core Ideas Long‐term (30 yr) diversified crop rotations had a positive impact on soil C fractions. Carbon fractions increased with increased rotation length. Diversified rotations enhanced soil C sequestration and microbial biomass and activity. Diversified crop rotations may enhance C fractions and soil quality by affecting the quality and quantity of crop residue returned to the soil compared with monocropping and fallow. We evaluated the effect of 30‐yr‐old diversified crop rotations on soil C fractions at 0‐ to 15‐, 15‐ to 30‐, and 0‐ to 30‐cm depths in the Loess Plateau of China. Crop rotations were continuous winter wheat (Triticum aestivum L.) (W); 3‐yr rotations of corn (Zea mays L.)–winter wheat–winter wheat–millet (Eleusine coracana L.) (CWWM), pea (Pisum sativum L.)–winter wheat–winter wheat–millet (PWWM), and sainfoin (Onobrychis viciifolia Scop.)–winter wheat–winter wheat–sainfoin (SWWS); 4‐yr rotation of pea–winter wheat–winter wheat–corn (PWWC); and 8‐yr rotation of alfalfa (Medicago sativa L.) (4 yr)–potato (Solanum tuberosum L.) (1 yr)–winter wheat (3 yr) (A4PoW3). A fallow (F) treatment was also included for comparison. Carbon fractions were soil organic C (SOC), particulate organic C (POC), potential C mineralization (PCM), and microbial biomass C (MBC). At all depths, SOC and POC were greater in A4PoW3 than F, W, and PWWC. The PCM and MBC were greater with SWWS than most other crops rotations. Soil C fractions increased linearly with increased belowground crop residue returned to the soil. Both SOC and POC increased linearly with increased rotation length. All C fractions were also correlated with mean annualized crop grain yield. Diversified crop rotations with increased belowground residue production enhanced soil C sequestration and microbial biomass and activity as well as crop yield compared with monocropping.

Decadal Nitrogen Fertilization Decreases Mineral‐Associated and Subsoil Carbon: A 32‐Year Study

AbstractCrop residues and manure are important sources of carbon (C) for soil organic matter (SOM) formation. Crop residue return increases by nitrogen (N) fertilization because of higher plant productivity, but this often results only in minor increases of SOM. In our study, we show how N fertilization and organic C additions affected SOM and its fractions within a 32‐year‐long field‐experiment at Puch, Germany. Five organic additions, no‐addition (control), manure, slurry, straw and straw + slurry, were combined with three mineral N fertilization rates (no, medium and high fertilization), which resulted in 1·17–4·86 Mg C‐input ha‐1 y‐1. Topsoil (0–25 cm) SOM content increased with N fertilization, mainly because of the C in free light fraction (f‐LF). In contrast, subsoil (25–60 cm) SOM decreased with N fertilization, probably because of roots' relocation in Ap horizon with N fertilization at the surface. Despite high inputs, straw contributed little to f‐LF but prevented C losses from the mineral‐associated SOM fraction (ρ &gt; 1·6 g cm‐3) with N fertilization, which was observed without straw addition. Above (straw) and belowground (roots) residues had opposite effects on SOM fractions. Root C retained longer in the light‐fractions and was responsible for SOM increase with N fertilization. Straw decomposed rapidly (from f‐LF) and fueled the mineral‐associated SOM fraction. We conclude that SOM content and composition depended not only on residue quantity, which can be managed by the additions and N fertilization, but also on the quality of organics. This should be considered for maintaining the SOM level, C sequestration, and soil fertility. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Fate of straw- and root-derived carbon in a Swedish agricultural soil

To maximise carbon (C) storage in soils, understanding the fate of C originating from aboveground and belowground residues and their interaction with fertiliser under field conditions is critically important. The use of 13C natural abundance provides unique opportunities to separate both C sources. We investigated the effect of 16 years of C3 straw and C4 root input, with and without nitrogen (N) addition, on SOC stocks and C distribution in soil fractions in the long-term frame trial at Ultuna, Sweden. The straw C input was fixed at 1.77 Mg ha−1 year−1, while the root input depended on maize plant growth, enabling studies on how N fertilisation affected (i) stabilisation of residues and (ii) plant C allocation to belowground organs. Four treatments were investigated: only maize roots (Control), maize roots with N (Control + N), maize roots and straw (Straw) and maize roots, straw and N (Straw + N). After 16 years, 5.6–8.9% of the total SOC stock in the 0–20 cm soil layer was maize-derived. In all four treatments, the relatively labile SOC fractions decreased, while the proportion of more refractory fractions increased. Based on allometric calculation of root inputs, retention of maize roots was 38, 26, 36 and 18% in the Control, Control + N, Straw and Straw + N treatments, respectively. The estimated retention coefficient of C3 straw in the Straw + N treatment was higher than that in the Straw-N treatment. We interpreted these results thus (1) roots were better stabilised in the soil than straw; (2) N fertilisation caused a shift in root to shoot ratio, with relatively more roots being present in N-deficient soil; and (3) N fertilisation caused greater stabilisation of residues, presumably due to increased microbial C use efficiency.

Use of 33P to trace in situ the fate of canola below-ground phosphorus, including wheat uptake in two contrasting soils

Our understanding of the contribution of crop root residues to phosphorus (P) cycling is mainly derived from studies using excavated roots re-introduced to soil. This study aims to quantify total below-ground P (BGP) of mature canola in situ and to estimate directly the proportion accessed by subsequent wheat. 33P-Labelled phosphoric acid was fed by stem wick to canola (Brassica napus) grown in sand or loam in pots. Shoots were removed from all plants at maturity. Half of the pots were destructively sampled. After a 3-week fallow, wheat was grown for 5 weeks in the remaining undisturbed pots. At canola maturity, 23–36% of the 33P was partitioned in recovered roots and 34–40% in the soil. More 33P was recovered in the loam than the sand. Within the soil, 6–10% of the fed 33P was present in resin P and 3–5% was in hexanol-released P pools. Ratios of shoot P : BGP (8 : 1 in sand and 15 : 1 in loam) were much narrower than those of shoot P : recovered root P (17 : 1 in sand and 39 : 1 in loam). A greater proportion and amount of the mature canola BG33P was recovered by wheat grown in the loam (26%, 2.6 mg/plant) than in the sand (21%, 1.5 mg/plant). The majority of canola BG33P remained in the bulk soil. Input of P below-ground by mature canola and subsequent P benefit to wheat was greater in loam than sand. The P from canola below-ground residues contributed up to 20% of P uptake in wheat during the first 5 weeks of growth. Longer term benefits of P from below-ground residues require investigation.