Kristian Thorup-Kristensen

Catch crops and green manures as biological tools in nitrogen management in temperate zones

During the last decades a lot of research have been made on the use of cover crops. Cover crops are grown for many purposes, but most of the resent interest have focused on their effects on nitrogen. Studies have been made on catch crops grown to catch N from the soil and prevent leaching losses to the environment and on legume green manure crops grown to improve the N supply for succeeding crops. Many of the experiments have been agronomic studies, where choise of plant species or management strategies have been tested to identify the optimal way to grow cover crops in a specific situation. Other experiments have aimed at gaining more basic understanding of the effects of catch crops or green manure crops on N dynamics. These studies include subjects as catch crop growth, root growth, N uptake and soil depletion, kill-date, N mineralisation and pre-emptive competition, and how these factors interact with soil, climatic conditions, and the main crops in the cropping system, both in the short term and in the longer term. Together, the results from these studies have given a more comprehensive understanding of the mechanisms by which a catch crop or a green manure affect N leaching losses and N supply for succeeding crops. The principles governing the effect of catch crops on N supply for succeeding crops have been found to differ basically from the effects N effects of added organic matter. This is mainly due to the fact that a catch crop do not add N to the soil, the N which is incorporated with the catch crop has first been taken from the soil. In the review, we discuss this new knowledge of catch crops and green manures, and how it helps us to understand why the effects obtained by catch crops are so variable. We also discuss how it can be used to develop strategies which will improve the results we obtain from catch crops and green manures, and to make them more predictable. Many studies have been made on other effects of cover crops, on soil borne diseases, pests, weeds, soil structure, erosion, soil biology, and other nutrients than N. Though there are many studies, they are scattered over a large number of themes, and research in cover crop effects in most of these themes can be said to be at a very early stage. However, many very interesting effects have been observed, an there seems to be a significant potential for development of cover cropping also for other objectives than improved N husbandry.

Are differences in root growth of nitrogen catch crops important for their ability to reduce soil nitrate-N content, and how can this be measured?

An experiment was made to measure root growth of nitrogen catch crops, to investigate whether differences in root growth among plant species are related to their ability to deplete the soil nitrate-N pool. Large differences were observed in root growth parameters. Monocot species had rooting depth penetration rates in the range of 1.0 to 1.2 mm d−1 °C−1, whereas the non-legume dicot species had rates between 1.5 and 2.3 mm d−1 °C−1. Substantial differences were also found in the lag time from sowing until significant root growth was observed. The estimated temperature sum needed for the crops to reach a rooting depth of 1.0 m varied from 750 d °C for fodder radish to 1375 d °C for Italian ryegrass. The depth distribution of the root system varied strongly, and at a depth of 1.0 m the non-legume dicot species generally had root intensities (number of root intersections m−1 line on the minirhizotrons) 12 times as high as the monocot species.The amount of nitrate left in the topsoil (0–0.5 m) was only weakly correlated to a few of the measured plant and root parameters, whereas nitrate left in the subsoil (0.5–1.0 m) was clearly correlated to several root parameters. Subsoil nitrate residues were well correlated to root intensity, but showed even stronger correlations to more simple estimates of rooting depth. In the deepest soil layer measured (1.0–1.5 m), the soil water nitrate concentration was reduced from 119 μg L−1 without a catch crop to 61 μg L−1 under Italian ryegrass and to only 1.5 μg L−1 under fodder radish.The results show that to identify the important differences in root growth among catch crops, root growth must be measured in deep soil layers. In this study, none of the measurements made aboveground or in the upper soil layers were well related to subsoil nitrate depletion.

Modelling and measuring the effect of nitrogen catch crops on the nitrogen supply for succeeding crops

AbstractNitrogen catch crops are grown to absorb nitrogen from the rooting zone during autumn and winter. The uptake of N (Nupt) from the soil inorganic N pool (Nmin) to a pool of catch crop nitrogen, will protect the nitrogen against leaching. After incorporation, a fraction (m) of the catch crop nitrogen is mineralized and becomes available again. However, not all available nitrogen present in the soil in the autumn is lost by leaching during winter. A fraction (r) of the nitrogen absorbed by the catch crop would, without a catch crop, have been retained within the rooting zone. The first year nitrogen beneficial effect (Neff) of a catch crop may then be expressed b Neff = m*Nupt - r* Nupt The soil-plant simulation model DAISY was evaluated for its ability to simulate the effects of catch crops on spring Nmin and Neff. Based on incubation studies, parameter values were assigned to a number of catch crop materials, and these parameter values were then used to simulate spring Nmin. The model was able to predict much of the vairiation in the measured spring Nmin (r2 = 0.48***) and there was good agreement between the measured and the simulated effect of winter precipitation on spring Nmin and Neff.Scenarios including variable soil and climate conditions, and variable root depth of the succeeding crop were simulated. It is illustrated that the effect of catch crops on nitrogen availability for the succeeding crop depends strongly on the rooting depth of the succeeding crop. If the succeeding crop is deep rooted and the leaching intensity is low, there is a high risk that a catch crop will have a negative effect on nitrogen availability. The simulations showed that the strategy for the growing of catch crops should be adapted to the actual situation, especially to the expected leaching intensity and to the rooting depth of the succeeding crop.

Uptake of15N labeled nitrate by root systems of sweet corn, carrot and white cabbage from 0.2–2.5 meters depth

Leaching of NO3− from vegetable cropping systems can be very high compared to arable systems. This is a problem for vegetable growers in general as it decreases groundwater quality, and for organic growers in particular as the organic production is often limited by N. In a field experiment, we investigated the N uptake and root growth of three vegetables using minirhizotrons reaching 2.4 m with the purpose to study the relationship between vegetable root distribution and uptake of NO3− from deep soil layers. NO3− uptake was studied over a 6 d period at the end of September by injection of 15 NO3− at four depths in the ranges: 0.2–0.8, 0.6–1.8, and 1–2.5 m under late sweet corn (Zea mays L. convar. Saccharata Koern.), carrot (Daucus carota L.), and autumn white cabbage (Brassica oleracea L. convar. capitata (L.) Alef. var. alba DC), respectively. The root depths of the three crops were 0.6, 1.3, and more than 2.4 m, respectively. Uptake of15N was close to zero from placements below root depth, and linear relationships were found between root density and15N uptake from different depths. N inflow rates (uptake per unit root length) were in the same range for all species and depths. This indicates that the very different N use efficiencies often found for vegetable crops depend on species specific differences in root development over time and space, more than on differences in N uptake ability of the single root. Thus deep rooting is important for deep N uptake. Knowledge about deep root growth enables design of crop rotations with improved N use efficiency based on re-cycling of deep soil NO3− by vegetables.

Disproportionately high N-mineralisation rates from green manures at low temperatures – implications for modeling and management in cool temperate agro-ecosystems

We examined the decomposition of Medicago lupulina, Melilotus alba and Poa pratensis at 3, 9, and 25 °C during 4 weeks. There was a strong temperature effect on the rate of CO2 evolution, and thus the extent of energy exhaustion from the added substrates. However, there was no concomitant retardation of N mineralisation at low temperatures. In the analysis of variance of mineralized N the residue type gave a 10 times larger contribution to the regression than the temperature (T), whereas for CO2 evolution residue type and temperature were equally important contributors. This indicates that although the temperature has a statistically significant effect on N-mineralisation it is substantially less than compared with the effect on carbon mineralisation in the materials examined. The retardation of carbon mineralisation was least strong in Melilotus alba that had a relatively low cellulose content, and a higher content of low molecular compounds. Though more research will be necessary to consolidate and explain this phenomena, it is likely that an important factor is a decrease in the bioavailability of C-rich polymers at low temperatures, and thus a preferential utilization of N-rich low molecular substances. Nitrification was not effectively deterred at 3 °C. Thus, in terms of management, it is pertinent to reconsider the timing of green manure and catch crop incorporation in cool temperate climate regions, since the rapid release of nitrogen, coupled with the relatively undeterred nitrification may result in a high N leaching risk by early incorporation, but a low risk for N immobilization at late incorporation, if N rich residues are used.

The Effect of Nitrogen Catch Crops on the Nitrogen Nutrition of a Succeeding Crop: I. Effects through Mineralization and Pre-emptive Competition

Abstract The effects of catch crops on the nitrogen nutrition of a succeeding carrot crop were investigated. An attempt was made to distinguish the effects of growth and nitrogen uptake by the catch crop from the effect of mineralization of its residues. It was found that growth and nitrogen uptake by catch crops could reduce the nitrogen supply to the succeeding carrot crop through pre-emptive competition, whereas mineralization of nitrogen from the catch crop residues increased the nitrogen supply to the carrot crop. Nitrogen uptake by the carrots was thus highest where catch crop residues were incorporated on plots where no catch crop had been grown. The effect of the mineralization was found mainly to influence topsoil mineral nitrogen contents and the early nitrogen uptake by carrots, whereas the effect of pre-emptive competition was to reduce subsoil mineral nitrogen content and the nitrogen uptake by carrots late in the growing season. The apparent recovery of catch crop nitrogen by carrots was bet...

N-Fixation of selected green manure plants in an organic crop rotation

ABSTRACT The aim of this study was to investigate the N-fixation potential of different leguminous green manure plants grown in the autumn after harvest of a barley main crop. Fixed above ground N derived from the atmosphere (Ndfa) was estimated both by the 15N isotope dilution method and by the total-N difference method. Winter rape (Brassica napus), winter rye (Secale cereale) and Italian ryegrass (Lolium multiflorum) were grown as non-fixing control plants for the estimation of N-fixation of leguminous green manure plants. It was concluded that Italian ryegrass was the most suitable control plant, and that the choice of control plant can be important for the results. When using Italian ryegrass as control plant, no significant difference was observed between the total N difference method and the 15N isotope dilution method. N-fixation varied strongly among the plant species and differed between the two years. Hairy vetch (Vicia villosa), crimson clover (Trifolium incarnatum) and Persian clover (Trifolium resupinatum) fixed more than 100 kg N ha−1 at least in one of the two investigated years. The highest estimated Ndfa was 149 kg N ha−1 in the above ground plant material of hairy vetch in 1997. Ndfa of common vetch (Vicia sativa) and Egyptian clover (Trifolium alexandrinum) was below 60 or 100 kg N ha−1 in 1996 and 1997, respectively. Weather conditions and soil moisture, which are crucial during germination and early plant development, may be an explanation for lower N- fixation in 1996 than in 1997. With respect to N, the results indicate that extended use of green manure could strongly reduce the need for full year green manure crops in stockless organic crop rotations.

Modelling diverse root density dynamics and deep nitrogen uptake—A simple approach

We present a 2-D model for simulation of root density and plant nitrogen (N) uptake for crops grown in agricultural systems, based on a modification of the root density equation originally proposed by Gerwitz and Page in J Appl Ecol 11:773–781, (1974). A root system form parameter was introduced to describe the distribution of root length vertically and horizontally in the soil profile. The form parameter can vary from 0 where root density is evenly distributed through the soil profile, to 8 where practically all roots are found near the surface. The root model has other components describing root features, such as specific root length and plant N uptake kinetics. The same approach is used to distribute root length horizontally, allowing simulation of root growth and plant N uptake in row crops. The rooting depth penetration rate and depth distribution of root density were found to be the most important parameters controlling crop N uptake from deeper soil layers. The validity of the root distribution model was tested with field data for white cabbage, red beet, and leek. The model was able to simulate very different root distributions, but it was not able to simulate increasing root density with depth as seen in the experimental results for white cabbage. The model was able to simulate N depletion in different soil layers in two field studies. One included vegetable crops with very different rooting depths and the other compared effects of spring wheat and winter wheat. In both experiments variation in spring soil N availability and depth distribution was varied by the use of cover crops. This shows the model sensitivity to the form parameter value and the ability of the model to reproduce N depletion in soil layers. This work shows that the relatively simple root model developed, driven by degree days and simulated crop growth, can be used to simulate crop soil N uptake and depletion appropriately in low N input crop production systems, with a requirement of few measured parameters.

Vertical and horizontal development of the root system of carrots following green manure

Cover crops grown as green manure or for other purposes will affect nitrogen (N) distribution in the soil, and may thereby alter root growth of a succeeding crop. During two years, experiments were performed to study effects of nitrogen supply by green manure on root development of carrots (Daucus carota L). Total root intensity (roots cm−2 on minirhizotrons) was significantly affected by the green manures, and was highest in the control plots where no green manure had been grown. Spread of the root system into the interrow soil was also affected by green manure treatments, as the spread was reduced where spring topsoil Nmin was high. Although N supply and distribution in the soil profile differed strongly among the treatments, no effect was observed on the rooting depth of the carrot crops. Across all treatments the rooting front penetrated at a rate of 0.82 and 0.68 mm day−1 °C−1 beneath the crop rows and in the interrow soil, respectively. The minirhizotrons only allowed measurements down to 1 m, and the roots reached this depth before harvest. Extrapolating the linear relationship between temperature sum and rooting depth until harvest would lead to rooting depths of 1.59 and 1.18 m under the crop rows and in the interrow soil respectively. Soil analysis showed that the carrot crop was able to reduce Nmin to very low levels even in the 0.75 to 1.0 m soil layer, which is in accordance with the root measurements. Still, where well supplied, the carrots left up 90 kg N ha−1 in the soil at harvest. This seemed to be related to a limited N uptake capacity of the carrots rather than to insufficient root growth in the top metre of the soil.

Root Development of Nitrogen Catch Crops and of a Succeeding Crop of Broccoli

Abstract The root development of seven catch crop species, and of a broccoli crop following catch crops, was followed by a minirhizotron technique. Large differences in root growth patterns were found among the catch crops. Fodder radish already had a rooting depth of 112 cm 49 days after sowing, whereas Italian ryegrass did not reach this depth until 175 days after sowing. The catch crops with a fast establishment of deep rooting also reduced the mineral nitrogen content in the subsoil most. Broccoli showed a significantly higher root density in the subsoil when grown after narrow-leaved lupin and lower after winter rape than where no catch crop had been grown. The main reason for these differences appeared to be the distribution of mineral nitrogen in the soil.