Below ground nitrogen dynamics in the sequence clover-grass maize in the DOK long term experiment

We investigated the effect of organic versus conventional cropping systems on the below ground nitrogen inputs of Trifolium pratense L., its transfer to corresponding grass and the fate in the soil organic matter in the clover-grass ley of the DOK long term experiment, Switzerland. BGN tended to be largest in conventional and organic treatments with standard fertilisation and decreased with lower fertilisation intensity. The largest amount of clover N transferred to grass was observed in the minerally fertilised conventional treatment. Clover N derived from rhizodeposition was rapidly stabilised in all treatments to clay rich fractions and thus clover N will have a relatively low direct N contribution to subsequent nonlegumes

Symbiotische N2 Fixierung und N–Bilanz von Soja unter Be-rücksichtigung der N-Rhizodeposition im DOK Versuch

Symbiotic N2 fixation (Nfix) and N-balance was determined from organic and conven-tional grown soybean in the DOK experiment (Switzerland). Nfix was calculated i) based solely on soybean aboveground N (AGN) and ii) additionally taking into account belowground N (BGN), comprising N in physical roots and N rhizodeposition. Nfix was averagely two times higher considering AGN and BGN, ranging from 22 g N m-2 (conventional manure) to 37 g N m-2 (conventional mineral). N balances were positive in all treatments, but with 2 – 7 g N m-2 based on AGN solely comparatively small. These values were exceeded fourfold when considering BGN ranging from 13 g N m-2 (conventional manure) to 21 g N m-2 (organic and conventional mineral)

New insights in below ground nitrogen of clover-grass mixtures

Estimates of symbiotic nitrogen fixation (SNF) of clover in mixtures usually consider only aboveground clover nitrogen (N). However, belowground inputs of clover N derived from SNF via roots and rhizodeposition and its transfer to associated grass may contribute significantly to the amount of symbiotically fixed clover N. A microplot study with a red clover ( L.)-perennial ryegrass ( L.) model mixture was conducted within zero fertilised, bio-organic and conventional field plots of the DOK (bio-Dynamic, bio-Organic, Konventionell) long-term experiment during two consecutive years

Nitrogen Budgets and Soil Nitrogen Stocks of Organic and Conventional Cropping Systems: Trade-Off between Efficiency and Sustainability of Nitrogen Use

Organic and conventional cropping systems differ in the nature and amounts of nitrogen (N) inputs, which may affect efficiency and sustainability of N use. In the DOK (bio-Dynamic, bio-Organic, Konventionell) field experiment, organic and conventional cropping systems have been compared since 1978 at two fertilization levels. Nitrogen inputs via manure and/or mineral fertilizers, and N exports from plots with harvested products have throughout been recorded. For all treatments, N outputs with harvests have exceeded the inputs with fertilizers. Over the past years, symbiotic N2 fixation by soybean and clover grown in the trial has additionally been assessed, indicating average annual inputs of about 100 kg ha-1 yr-1 of N fixed from the atmosphere. Soil surface budgets opposing N inputs via fertilization, symbiotic fixation, seeds and deposition to N outputs via harvested products have been computed at the plot level for the duration from 1985 to 2012. The resulting balances range from negative values of about -20 kg N ha-1 yr-1 (where outputs exceed the sum of said N inputs) to surpluses of about +50 kg N ha-1 yr-1. The budget based N use efficiency (NUE; N output via harvested products divided by sum of N inputs) in the case of negative balances suggests irrationally high NUE (>100%), while positive balances are related to lower NUE for treatments with inputs exceeding outputs. Negative balances, however, indicate soil N mining, while surpluses point to a risk of N losses, and/or N accumulation in the soil. Estimation of soil N stock changes based on yearly total N concentration measurements in the topsoil layer is currently ongoing. Preliminary results suggest that soil N stocks in the topsoil decreased under all treatments more than expected from the N balance, and that positive N balances are needed to maintain topsoil N stocks. An increase in soil N concentration was observed in none of the treatments. In conclusion, the results indicate an efficiency-sustainability trade-off. Treatments with a higher NUE lose more soil stock N than those with a lower NUE. Treatments with lower NUE indicate higher N losses from the studied crop-topsoil system. Sustainable soil N management in addition to organic fertilizer inputs might at this site require reduced soil tillage. The significance of N contained in deeper soil layers, and deep rooting crops in recovering leached N should as well be investigated

Wurzelbürtige Stickstoff- und Kohlenstoffeinträge in den Boden: Erkenntnisse aus sieben Jahren DOK Wurzelforschung

We investigated nitrogen and carbon below ground inputs of soybean, clover-grass and maize and management effects of biological and conventional systems in the DOK long term experiment. We found for soybean increasing below ground C and N inputs with decreasing fertilization intensity whereas those effects were small for clover-grass. Below ground nitrogen inputs of red clover could be estimated by above ground N multiplied with a factor of 0.5

Stickstoffbilanzen in biologischen und konventionellen Anbausystemen Das Effizienz-Nachhaltigkeits-Dilemma

N-balances over 35 years from the DOK trial are presented and combined with Nstock changes in DOK treatments on different fertilisation levels. Results strongly indicate an N efficiency-sustainability dilemma: DOK treatments with a high nitrogen use efficiency (NUE) lose more soil stock N than those with a lower NUE but higher N losses from the system. The biodynamic system showed little advantage in terms of soil N stocks sustainability while the solely mineral fertilised conventional treatment had highest NUE across all inputs including soil N change

The Multifunctional Challenge Of Future Agriculture – Answers From 40 Years Dok Research

"Achieving sufficient and stable crop yields with limited cropland and without excessive use of non-renewable resources under a changing climate are the multifunctional challenges of future agriculture. We compared the performance and sustainability of organic and conventional cropping systems in the DOK long-term systems comparison after 40 years of management. For the first time we present a comprehensive evaluation of the whole DOK design including the systems with reduced stocking rates. Yield, nutrient dynamic and soil quality evaluations show clearly the trade-offs between productivity and sustainability in organic as well as in conventional systems. Low input conventional systems reveal the best input-output performance but lowered soil quality; regular organic systems were most sustainable but achieved only moderate non-legume yields.

Red clover belowground N input affected by the nutrient availability and its fate during two years of clover-grass sward cultivation

Di nitrogen (N) fixation of legume-rhizobia symbiosis represents a relevant N source for agricultural systems and has the potential to reduce the input of synthetic N fertiliser. Legumes in mixture with grass constitutes an important component of cropping systems of crop-livestock farms. In recent years, these mixtures became an interesting alternative in stockless farms as feedstock for biogas production. Clover growing in association with grass receives 80% of its N from symbiotic N2 fixation. Estimations of fixed N have usually considered neither belowground clover N nor clover N transferred to the associated grass. One reason might be that the assessment of belowground N is laborious, since roots have to be recovered from the soil and N derived from rhizodeposition has to be estimated. Rhizodeposition N, composed of particulate and non-particulate compounds, is released into the soil from living roots via exudation and root turnover. Rhizodeposition has been studied by stable isotope 15N enrichment techniques. Likewise, N transfer from clover to grass has been assessed with 15N techniques using both the 15N enrichment and 15N natural abundance method. Fixed N contained in above- and belowground N of clover-grass swards potentially presents a significant N input in both conventional and organic cropping systems. These systems differ in fertilisation and crop protection strategies. The amount and the quality of applied fertilisers result in differences in soil nutrientavailability. Organically cropped soils often have a greater soil microbial biomass both in size and in activity compared to conventionally cropped ones. There is a lack of understanding on how these factors affect the input of symbiotically fixed N and on the fate of legume N remaining in the soil. To examine these relations, a microplot study was conducted in the DOK long-term field experiment, since 33 years of organic vs. conventional cropping with respective fertilisation and plant protection strategies resulted in soils with a gradient in nutrient-availability, soil microbial biomass both in size and in activity, and 15N natural abundance of soil N. Red clover (Trifolium pratense L.) above- and belowground N was quantified in model clover-grass swards. Swards were cultivated in microplots over two consecutive years. Microplots were located in DOK field plots with different fertilisation strategies. In general, the fertilisation increased in the order zero-fertilisation control (NOFERT), organic cropping system with half dose fertilisation (BIOORG1), organic cropping system with regular dose fertilisation (BIOORG2), and conventional cropping system (CONMIN2) with regular dose fertilisation. The organic cropping system at level 1 and 2 was fertilised with farmyard manure (manure and slurry) while the conventional cropping system was supplied by mineral fertiliser exclusively. Red clover rhizodeposition N was determined by 15N multiple-pulse leaf labelling (chapter 1). All roots were carefully removed from the soil. Subsequently, the fate of N derived from rhizodeposition being still present in the soil was investigated at different points of time using a sequential extraction. This procedure allows to extract soluble N and microbial biomass N from the soil. The remaining soil organic matter pools were separated in low and high density occluded particulate organic matter and silicate and quartz mineral-organic associations using a sequential density fractionation. Cropping system related effects from the size and activity of the microbial biomass on the General introduction 4 incorporation of N derived from rhizodeposition in soil pools and soil organic matter pools were assessed (chapter 2). Furthermore, different 15N natural abundance and 15N enrichment procedures were compared to determine the transfer from red clover N to perennial ryegrass (Lolium perenne L.). Subsequently, the 15N natural abundance method was verified with cropping systems under identical environmental and management conditions but differing in 15N natural abundance of the applied N fertilisers. One 15N natural abundance procedure was selected to quantify red clover N and symbiotically fixed red clover N being transferred to perennial ryegrass (chapter 3). Finally, the results from chapter 1 to 3 were scaled up from the microplot level to the field plot level of the DOK experiment. For this, N yields and clover proportions of clover-grass swards of field plots were used. Results were synthesised to present an overall picture on symbiotically fixed N in the plant-soil system of clover-grass swards under different cropping systems (general discussion). Red clover belowground N developed proportional to aboveground N at a ratio of 0.4 to 1 irrespective of the fertilisation strategy and the time of cultivation. However, the ratio N derived from rhizodeposition to root N changed with time. More than 90% of rhizodeposition N was incorporated into soil organic matter pools. Thereof, about 40% was found in silicate mineral-organic associations, 40% to 50% in both occluded particulate organic matter fractions, and the remainder in quartz mineralorganic associations irrespective of the fertilisation strategy related organic matter input. The proportions of grass N transferred from red clover N being estimated by 15N natural abundance procedures were within the range of that being estimated by 15N enrichment based procedures. About 40% of perennial ryegrass N was transferred from red clover in the low N fertilised swards (< 50 kg mineral N ha-1 a-1) NOFERT, BIOORG1 and BIOORG2. This corresponds to more than one third of perennial ryegrass N if related to symbiotically fixed red clover N. Up scaled from the microplot level to the field plot level of the DOK experiment, the sward plant-soil system obtained 300 to more than 400 kg N ha-1 from the symbiotic N2 fixation of clover after two years of cultivation. From that symbiotically fixed clover N, nearly 50% was present in the clover yield, about one third was transferred to grass, nearly 15% was accumulated in soil organic matter pools, 5% remained in the stubble and the root, and less than 1% was found in the both soil pools soluble N and microbial biomass N. Subsequent to the last harvest at the end of the 2nd year, on average 100 kg ha-1 symbiotically fixed clover derived N remained in the stubble and the roots of clover and grass and in the soil pools. Remaining N increased in the order NOFERT, CONMIN2, BIOORG2, and BIOORG1 at a ratio of 0.9 to 0.9 to 1.1 to 1.2 related to the average of the four cropping systems. In conclusion, red clover belowground N could be estimated from aboveground N by a factor of 0.4 irrespective of the fertilisation strategy and the cultivation time. Allocation of red clover rhizodeposition N to the silicate mineral-organic association was the dominant process N derived from rhizodeposition remaining in the soil was exposed. This process proceeded within months, but remained unaffected by the cropping system related microbial size and activity. 15N natural abundance procedures resulted in somewhat less variable estimates of N transfer from red clover to perennial ryegrass than 15N enrichment General introduction 5 procedures. Cropping systems affected the amount of red clover and perennial ryegrass N accumulation, red clover N derived from rhizodeposition, and N transfer to grass, but hardly affected the partitioning of N derived from rhizodeposition to soil pools. The partitioning of red clover N between above- and belowground and the partitioning within soil organic matter pools was not affected by cropping systems in the studied plant-soil system. Up scaled from the microplot to the field plot level of the DOK experiment, about 50% of symbiotically fixed clover N was deposited into the soil (≈15%) or transferred to grass (≈35%) at the end of the 2nd cultivation year. Hence, clover belowground N and N transfer to grass may represent important and considerable N inputs to agricultural systems

Overestimation of crop root biomass in field experiments due to extraneous organic matter

Root biomass is one of the most relevant root parameters for studies of plant response to environmental change, soil carbon modeling or estimations of soil carbon sequestration. A major source of error in root biomass quantification of agricultural crops in the field is the presence of extraneous organic matter in soil: dead roots from previous crops, weed roots, incorporated above ground plant residues and organic soil amendments, or remnants of soil fauna. Using the isotopic difference between recent maize root biomass and predominantly C3-derived extraneous organic matter, we determined the proportions of maize root biomass carbon of total carbon in root samples from the Swiss long-term field trial “DOK.” We additionally evaluated the effects of agricultural management (bio-organic and conventional), sampling depth (0–0.25, 0.25–0.5, 0.5–0.75 m) and position (within and between maize rows), and root size class (coarse and fine roots) as defined by sieve mesh size (2 and 0.5 mm) on those proportions, and quantified the success rate of manual exclusion of extraneous organic matter from root samples. Only 60% of the root mass that we retrieved from field soil cores was actual maize root biomass from the current season. While the proportions of maize root biomass carbon were not affected by agricultural management, they increased consistently with soil depth, were higher within than between maize rows, and were higher in coarse (>2 mm) than in fine (≤2 and >0.5) root samples. The success rate of manual exclusion of extraneous organic matter from root samples was related to agricultural management and, at best, about 60%. We assume that the composition of extraneous organic matter is strongly influenced by agricultural management and soil depth and governs the effect size of the investigated factors. Extraneous organic matter may result in severe overestimation of recovered root biomass and has, therefore, large implications for soil carbon modeling and estimations of the climate change mitigation potential of soils

Above- and belowground nitrogen distribution of a red clover-perennial ryegrass sward along a soil nutrient availability gradient established by organic and conventional cropping systems

Aims Belowground legume nitrogen (N) composed of roots and rhizodeposition is an important N input to soils, but published data of belowground N vary broadly, probably due to extrapolation from short-term experiments and dissimilar growing conditions. We quantified belowground N inputs of red clover (Trifolium pratense L.) during two consecutive years in a clover-grass sward along a soil nutrient availability gradient. Methods We established a red clover-perennial ryegrass (Lolium perenne L.) model sward in microplots located in field plots of the DOK experiment, which has a 33-year history of organic and conventional cropping, resulting in a soil nutrient availability gradient. Four treatments were examined: the zero fertilisation control, bio-organic with half and full dose manure application, and the conventional system with mineral fertilisation at full dose. We studied the development of clover aboveground and belowground N using multiple pulse 15N urea leaf labelling. Results Belowground clover N increased over time and with rising nutrient availability and was proportional to aboveground clover N at all times. Belowground clover N amounted to 40% of aboveground clover N during two consecutive years, irrespective of the nutrient availability status. Belowground clover N development was initially dominated by fast root growth, followed by enhanced root turnover during the second year. Potassium availability limited clover growth and total N accumulation in treatments with low nutrient availability. Conclusions Belowground red clover N inputs could be estimated from aboveground N by a constant factor of 0.4, regardless of the nutrient availability and cultivation time. Root turnover led to a distinct absolute increase of N rhizodeposition over time. Hence, N rhizodeposition, with an 80% share of belowground N, was the predominant N pool at the end of the second year