Improving N cycling in agroecosystems is one of the key challenges in
reducing the environmental footprint of agriculture. Further, uncertainty in
precipitation makes crop water management relevant in regions where it has
not been necessary thus far. Here, we focus on the potential of
winter-killed catch crops (CCs) to reduce N leaching losses from N mineralization
over the winter and from soil water management. We compared four single CCs (white mustard, phacelia, Egyptian clover and bristle oat) and two
CC mixtures with 4 and 12 plant species (Mix4 and Mix12) with a
fallow treatment. High-resolution soil mineral N (
Catch crops (CC, also named cover crops) are well-known tools for biological
N management in arable soils of temperate climates
(Thorup-Kristensen et al., 2003). Catch crops efficiently deplete
residual soil mineral N (
The potential of a certain CC for soil
The advantage of winter-killed CCs over winter-hardy is the saving
of energy for CC termination and the reduction of soil tillage
(Gollner et al., 2020). Even no-till practices without
herbicide applications are possible if CCs are reliably terminated by frost
(Romdhane et al., 2019). The challenge of
winter-killed CCs, however, is the risk of losing early N by leaching and
reducing the N carry-over to the succeeding crops if the plant residues are
mineralized too fast. The
Soil water management is becoming a new challenge with increasing aridity due to climate extremes and drought spells. Catch crops require water for their establishment and deplete the soil water pools while they grow. Competition for water with the main crop could result in yield reduction in semiarid and arid environments (Unger and Vigil, 1998). In contrast, maintaining CC residues on the soil surface reduces evapotranspiration and increases the infiltration capacity of the soils. Therefore, the net effect of a CC on the soil water balance depends on the standing time, precipitation timing and capacity of the CC to reduce water losses (Bodner et al., 2007; Unger and Vigil, 1998).
The objectives of this study were to investigate the N uptake efficiency in
monoculture vs. mixed CC stands and explore the timing of N release into the
soil after CC termination. Further, this study sought to evaluate the role of
different CC treatments on crop water availability in comparison to fallow
conditions. We followed the soil
The study was conducted at a long-term field experiment in Asendorf,
northern Germany (49 m above sea level, 52
Seven treatments in three replicates were investigated in a randomized block
design with four pure CCs, two mixtures and a fallow with no CC. Total plot size was 9 m by 9 m. The pure stands were white mustard
(
Catch crop plant biomass was determined at the end of the vegetation period
(9 November 2018) in three replicates per plot in 1 m wide squares. In
mixtures, plants were sorted according to the species and weight separately.
Dry matter yield (DMY) was determined gravimetrically after drying of plant
material at 60
Silage maize yield was determined from three individual subplots (1.5 m
Soil samples were taken with a manual soil corer from four soil depths
(0–10, 20–30, 50–60 and 70–80 cm). The sampling campaign comprised nine
sampling points in the period from 15 August 2018 to 24
July 2019 (Fig. S2, all dates provided in Table 2). Soil samples were
immediately stored in a cooler and transported to the laboratory for fresh
sample extraction. Approximately 10 g of soil material was extracted with
40 mL of a 0.0125 M CaCl2 solution after being shaken overhead for 60 min. Soil
As a complement to our offline soil moisture measurements, we installed data loggers for continuous monitoring. We used CS650 multiparameter sensors connected to a CR300 data logger (Campbell Scientific, Inc., Logan, USA). The logger delivered hourly data on soil volumetric water content, temperature and bulk electrical conductivity for the upper 0 to 30 cm soil depth. In the following, we discuss the data on volumetric water content only. The loggers collected continuous data for 1 year (from 27 September 2018 to 24 September 2019) with short removals for soil preparation. Unfortunately, some loggers were seriously damaged by wild animals and we lost six replicates for the statistical evaluations.
In total, 840
Total shoot biomass was highest for phacelia, followed by oat, the two
mixtures and mustard (Table 1). Clover showed less than half of the shoot
biomass of the other CC treatments. Root biomass summarized to 100 cm soil depth was highest for oat and decreased in the order Mix12, Mix4,
phacelia, mustard and clover. Total plant biomass followed a similar
pattern. The slow biomass development of clover encouraged voluntary winter
wheat and several regional weeds to germinate. These were weeded manually
once. The rest of the CC treatments suppressed weeds very well. Growth of
weeds and volunteer winter wheat was documented and their contribution to
total biomass was negligible. The highest average biomass N was found in
Mix12, but differences with other treatments were not significant due to large
data variability (Table 1). Despite the low biomass production, clover
produced remarkably high biomass N, in the same range as mustard. The
biomass
Plant biomass data of shoots, roots and total biomass from
different CC treatments. Mean values of six plots per treatment and
standard error (SE) are shown. Small letters (Sig) represent contributions
to significantly different groups at
Calculations of the total soil water content showed that in all depth increments, the field capacity of the soil was never exceeded in the soil profile (Fig. S3). A period of higher rainfall starting at the end of January (Fig. S1) increased the soil water content in the upper 10 cm from 66 % to 95 % of field capacity.
Total soil water storage was calculated from the summarized volumetric water
content to 80 cm soil depth, with the mean values of fallow as
a reference (100 %) in Fig. 1. The periods of CC growth clearly deplete
the soil water storage until its maximum growth. At the beginning of
November 2019, relative moisture levels under CC reached their minimum,
between 17 % and 33 % below the water storage of fallow, which was
significant for all CCs except clover (Table S3). The soil water storage started to recover already before CC
termination, and increased towards
the fallow level until mid-December 2018 (Fig. 1). Thereafter, all CC
treatments showed significantly higher soil water storage than the fallow
treatments (Table S3). After soil preparation and maize sowing, all CCs
still showed higher soil water storage than the fallow CCs (mustard
Volumetric soil water content down to 80 cm soil depth, with fallow as a 100 % reference (red horizontal line). Blue areas mark soil water content below the fallow level and yellow/brown values above. Continuous black lines were achieved by logistic regression models from three replicates, and the differences between the treatments at the individual sampling dates are provided in Table S3. The blue dotted line marks the termination of CC, and the vertical dashed line marks the seeding date of the main crop maize.
The loggers delivered 1 year of continuous data of the volumetric water
content to 30 cm soil depth and supported the results from our soil sample
measurements. Due to the loss of replications, we were not able to apply
proper statistical comparisons
of treatments. Nevertheless, the data fit the measurements of the volumetric
water content in the laboratory and allowed us to extend the interpretation
beyond maize harvest. In agreement with the laboratory measurements, the
logger data showed depletion of soil water by CC of all treatments (Figs. S4
and S5). Soil water was restored until soil preparation for maize seeding,
and exceeded the fallow levels except for mustard (Fig. S4; mustard
In Figs. 2 to 4, the dynamics of
Spatiotemporal resolution of
Clover was the only CC treatment that showed a similar pattern as bare
fallow (Fig. 2a and c). The variability between the maxima and minima,
however, was not as strong as in the fallow. Despite this, clover reduced
the average
Pairwise comparison of soil mineral N stocks (
Spatiotemporal resolution of
Spatiotemporal resolution of
Modelling of soil mineral N stocks
(
The quantification of the N leaching from downward migration with the
hydraulic gradient was not straightforward. However, we could estimate the
leaching from fallow and clover by simple calculation of the difference between
The DMY of silage maize was significantly higher than the fallow for all CC treatments except phacelia (Fig. 6). The Mix12 showed the highest average DMY, which was significantly higher than clover, oat and phacelia, but not statistic significant compared to mustard and Mix4.
Silage maize yield following different CC treatments from Asendorf field site in 2019. Small letters denote significant differences between CC treatments.
Catch crops deplete the available soil water during their growth (Fig. 1,
Fig. S5). At the same time, the convective transport rate of pore water
throughout the soil column is lower as under bare fallow conditions. The lower seepage
water rate causes a reduction of N leaching losses in humid years. The
extreme drought throughout 2018 did not result in seepage during the winter
of 2018/2019, as winter precipitation was not enough to restore the field
capacity of the soils (Fig. S3). Thus, the convective transport of water by
the mass flow through the soil column was absent. Despite the missing
seepage, our data indicate
Our data showed that in mid-December 2018 (4 weeks after CC termination), the soil water budget of CC was equal to the fallow budget and exceeded their level thereafter (Fig. 1). We assume the combination of several factors as drivers for constantly higher soil water content by all CC variants. (I) The exposed bare soil of the fallow is prone to splash effects by precipitation. Soil pores can be clogged, and surface crust formation restricts water infiltration into the soil, causing surface runoff. Catch crop residues increase soil roughness, protect the soil surface from splash effects and minimize surface runoff (Unger and Vigil, 1998). (II) Catch crops produce biopores by decaying roots. Preferential flow paths follow these root channels (Kautz, 2015) and increase water infiltration. (III) The litter of frost-killed CCs acts as mulch and reduces surface dry out by wind and interception losses (Unger and Vigil, 1998; Bodner et al., 2007, 2015).
However, not all CC species are equally suitable for water conservation. Species vary in their transpiration rates, water use efficiency, rooting depth and canopy coverage. Bodner et al. (2007) demonstrated that phacelia and vetch had a substantially lower evapotranspiration than rye and mustard. Within their study, mustard showed by far the largest transpiration rates. Our data indicate that clover consumed only half of the water during its growth period as compared to the other CCs. This was due to the lower biomass and rooting depth (Table 1; Heuermann et al., 2019). The soil of the upper 0 to 30 cm following clover constantly showed a 10 % to 33 % higher water content than fallow during the whole maize cropping period (median 19 %, Fig. S5). Mix12 and Phacelia showed similar results until mid-July. When maize plants entered the generative stage, all CC treatments except clover showed up to 200 % higher soil water content than the fallow treatments (Fig. S5). Similar results of higher crop water availability for the following crop were reported by Kaye and Quemada (2017) and were explained by a stronger mulch effect and higher soil organic matter (OM) content. Interestingly, in our experiments, clover showed the least benefit for crop water availability during the generative maize stage. The only explanation we came up with thus far could be the lower root and shoot biomass of clover that produced less particulate OM and biopores compared to the other CC treatments. This could result in a lower mulch effect and slower infiltration in the upper 30 cm of the clover treatments. The reasons that caused the differences between clover and other CC treatments remain quite speculative. Infiltration experiments, measurements of evaporation rates, aggregate fractionation or evaluation of biopores could help future studies explore the potential of CCs for soil water management.
Overall, CCs deplete soil water while they are growing and preserve water when they are killed, and their residues cover the ground. Catch crops can thus be used as tools for soil water management. In drought years, early frost killing or termination by rolling, crimping or mulching will stop transpirational water loss from the soil. Further, the residual mulch will decrease evaporation and increase infiltration. In moist or normal years, winter-hardy CCs and late termination can be used to dry the soil to a level that is optimal for seed bed preparation (Kaye and Quemada, 2017). Phacelia and Mix12 showed the most consistent results concerning the temporal course of the water content. We thus conclude that CC mixtures with high diversity combine several positive effects for soil water management for the following crop.
This study clearly visualizes
Clover reduced the soil
For mustard, we observed a very early flush of
The lowest
Nitrogen losses during CC rotations can also occur as greenhouse gas losses in
the forms of NH
The mixing of different CC species can compensate for the weaknesses
of individual single CCs with respect to residual N depletion,
extension of winter mineralization, and N transfer towards the following
maize crop. We demonstrated that CC mixtures with up to 22 % legumes in
shoot biomass took up similar amounts of
The
This study demonstrates that catch cropping during years with reduced soil water availability did not result in water shortages for the following crop. Catch crops deplete soil water while they are growing but reduce evaporation and preserve water compared to bare fallow after their dieback. The shallow incorporation of CC residues into the soil increased the infiltration and water storage capacity under the following main crop. The CC biomass and the percentage of soil cover from their residues determine the magnitude of the benefits for the following crop. Phacelia and Mix12 showed the most consistent combined results from data loggers and soil moisture measurements in the laboratory. High diversity mixtures most likely combine several positive aspects of soil water management that result from an optimized rooting depth and volume, plant transpiration, soil coverage by mulch and evaporation and OM input.
All CC treatments depleted the soil
All metadata and R codes required to reproduce the content of this study are publicly
available under Creative Commons Attribution 3.0 Germany. The files are
accessible from the Zenodo archive at the following DOI:
The supplement related to this article is available online at:
NG planned the research activity, participated in fieldwork and provided the statistical evaluation and R codes. StS, DH, RK, BB and DS participated in field and laboratory work and provided metadata. JB, NvW, UF and GG were involved in conceptualization, funding acquisition, project coordination and supervision. NG prepared the manuscript with contributions from all authors.
The contact author has declared that neither they nor their co-authors have any competing interests.
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
This work is part of the BonaRes (Soil as a Sustainable Resource for the Bioeconomy) project CATCHY (Catch-cropping as an agrarian tool for continuing soil health and yield increase). We are grateful to Silke Bokeloh and the whole laboratory team from the Institute of Soil Science, Leibniz Universität Hannover, for assistance with sample preparation and measurements.
This research has been funded by the Bundesministerium für Bildung und Forschung (grant no. 031A559C).The publication of this article was funded by the open-access fund of Leibniz Universität Hannover.
This paper was edited by Rafael Clemente and reviewed by two anonymous referees.