--- authors: - givenNames: - Norman familyNames: - Gentsch type: Person - givenNames: - Jens familyNames: - Boy type: Person - givenNames: - Juan Daniel familyNames: - Kennedy Batalla type: Person - givenNames: - Diana familyNames: - Heuermann type: Person - givenNames: - Nicolaus familyNames: - von Wirén type: Person - givenNames: - Dörte familyNames: - Schweneker type: Person - givenNames: - Ulf familyNames: - Feuerstein type: Person - givenNames: - Jonas familyNames: - Groß type: Person - givenNames: - Bernahrd familyNames: - Bauer type: Person - givenNames: - Barbara familyNames: - Reinhold-Hurek type: Person - givenNames: - Thomas familyNames: - Hurek type: Person - givenNames: - Fabricio familyNames: - Camacho Céspedes type: Person - givenNames: - Georg familyNames: - Guggenberger type: Person datePublished: value: '2020-05-23' type: Date dateReceived: value: '2020-02-05' type: Date dateAccepted: value: '2020-05-03' type: Date title: >- Catch crop diversity increases rhizosphere carbon input and soil microbial biomass isPartOf: volumeNumber: '56' isPartOf: title: Biology and Fertility of Soils identifiers: - name: doi propertyID: 'https://registry.identifiers.org/registry/doi' value: https://doi.org/10.1007/s00374-020-01475-8 type: PropertyValue publisher: name: 'Oxford University Press' type: Organization type: Periodical type: PublicationVolume --- ## Abstract Catch crops increase plant species richness in crop rotations, but are most often grown as pure stands. Here, we investigate the impacts of increasing plant diversity in catch crop rotations on rhizosphere C input and microbial utilization. Mustard (Sinapis alba L.) planted as a single cultivar was compared to diversified catch crop mixtures of four (Mix4) or 12 species (Mix12). We traced the C transfer from shoots to roots towards the soil microbial community and the soil respiration in a 13C pulse labelling field experiment. Net CO2-C uptake from the atmosphere increased by two times in mix 4 and more than three times in mix 12. Higher net ecosystem C production was linked to increasing catch crop diversity and increased belowground transfer rates of recently fixed photoassimilates. The higher rhizosphere C input stimulated the growth and activity of the soil microbiome, which was investigated by phospholipid fatty acid (PLFA) analyses. Total microbial biomass increased from 14 to 22 g m−2 as compared to the fallow and was 18 and 8% higher for mix 12 and mix 4 as compared to mustard. In particular, the fungal and actinobacterial communities profited the most from the higher belowground C input and their biomass increased by 3.4 and 1.3 times as compared to the fallow. The residence time of the 13C pulse, traced in the CO2 flux from the soil environment, increased with plant diversity by up to 1.8 times. The results of this study suggest positive impacts of plant diversity on C cycling by higher atmospheric C uptake, higher transport rates towards the rhizosphere, higher microbial incorporation and prolonged residence time in the soil environment. We conclude that diversified catch crop mixtures improve the efficiency of C cycling in cropping systems and provide a promising tool for sustainable soil management. ## Introduction Sustainable agriculture poses the challenge of developing innovative farming solutions to maintain or even increase productivity without negatively affecting the ecosystem services provided by agricultural landscapes. Catch cropping was identified as an economical and suitable tool for the ecological intensification of cropping systems (Kaye and Quemada 2017; Wittwer et al. 2017; Abdalla et al. 2019). Catch crops, also referred as cover crops, can be integrated between two main crops instead of having a fallow period and can be incorporated into the soil as green manure to fertilize the succeeding crop. The major goal of this practice is the improvement of soil quality and the prevention of soil erosion or leaching losses of postharvest nutrients (Thorup-Kristensen et al. 2003). Therefore, catch crops are valuable tools for nutrient management in crop rotations as they tighten nutrient cycles and improve nutrient use efficiency in agricultural landscapes. Catch crops are able to take on an important role in improving agrobiodiversity and microhabitat restoration (Tsiafouli et al. 2015; Vukicevich et al. 2016). Traditional catch crop systems often display reduced functional diversity by using only a small variety of catch crops mostly grown in pure stands. However, the utilization of catch crops that consist of only one or two species may limit the potential benefits that could be derived from this practice. With the application of diversified catch crop mixtures, broader functional plant performance and thus additional ecosystem services could be achieved. Combining legume and non-legume species in catch crop mixtures, for example, resulted in both high residual nutrient acquisition and green manure services for the following crop (Couëdel et al. 2018). In order to achieve the maximal benefits of catch cropping, one of the basic aims is the production of a high biomass yield. Increasing shoot biomass was positively related to weed suppression, prevention of nitrate leaching, biomass N and increasing OC and TN contents in soil (Mazzoncini et al. 2011; Finney et al. 2016). Increasing catch crop diversity, however, has not resulted in higher shoot biomass yields than those of high yielding sole catch crops so far (Finney et al. 2016; Heuermann et al. 2019). However, the green manure effects on the subsequent cash crop do not necessarily depend on aboveground biomass maximization. Rather, the quality of the litter residues, namely, a lower C/N ratio in litter, appears to have the highest effect on crop yield services (Finney et al. 2016; Couëdel et al. 2018). Combining legume and non-legume species is therefore a valuable tool for adjusting catch crop shoot biomass quality and mineralization patterns during subsequent cash crop growth. Furthermore, synergistic effects between plants, such as N facilitation (neighbouring plants benefit from the additional N input from N2-fixing symbionts), impact plant growth performance and biomass yields (Faget et al. 2013). But also in the absence of legumes, positive relationships between plant diversity and biomass productivity have been demonstrated (Van Ruijven and Berendse 2009). Perhaps the most important advantage of mixed catch crops comes from the rhizosphere. However, root performance in sole versus mixed plantings and its impact on nutrient acquisition have been poorly studied thus far. Our latest research demonstrated that catch crop mixtures overyield root biomass than that in single-species plantings (Heuermann et al. 2019). This higher root biomass yield was characterized by inter-species competition, where some species responded with higher root/shoot ratios but also with synergism between species and root growth. The same lack of information exists about the role of catch crop rhizodeposits and the effect of rhizosphere C input on the soil microbiome and C sequestration. Approximately 40% of the photosynthates produced by plants are translocated towards the rhizosphere within hours (Kumar et al. 2006). Rhizodeposition is closely related to root biomass, and for a few Brassica species, it was shown that catch crops store more N belowground when rhizodeposits were considered (Kanders et al. 2017). Root exudates enhance microbial activity and provide exoenzymes that mobilize nutrients from the soil (Kumar et al. 2006). Root mucilage, for example, is an important C source for soil microorganisms and increase their drought stress resistance (Ahmed et al. 2018). The C and N supply of roots to microbial utilization and nutrient mobilization depends on the plant species and was found generally higher for legume than for non-legume plants (Wichern et al. 2007). Increasing the rhizosphere input from the application of catch crop mixtures could therefore tighten element cycling in soil and stimulate C sequestration. Soil microorganisms provide essential ecosystem services in farmland soils such as organic matter (OM) decomposition, nutrient cycling or maintenance of soil structure (Paul 2007; Bender et al. 2016). Plants and microorganisms have developed intimate relationships helping them to exchange nutrients, maintain plant health and productivity (Chaparro et al. 2012). A recent synthesis highlighted the overall impacts of catch/cover crops on the microbial community in a wide range of arable soils (Kim et al. 2020). The meta-analyses found that catch crops increase the overall abundance, activity and diversity of soil microorganisms. But also the soil microbial functional diversity and their substrate use efficiency showed a strong positive response to catch crop treatments (Nivelle et al. 2016). The plant species applied as catch crops are important drivers of the soil microbiome and the selection of desired species or mixtures have been suggested as tools for their management (Finney et al. 2017). Catch crop research during recent decades has focused mostly on the performance of single species, while species diversity has remained poorly considered. The aim of the study was to test how catch crop diversity affects C cycling from the atmosphere through the plant-soil-microbiome system. More specifically, the efficiency for C uptake and the belowground input of recently photosynthesized C products and their partitioning in the rhizosphere was investigated. The allocation and incorporation of C compounds in belowground compartments is well known to occur within hours after its assimilation and the turnover of rhizosphere products spans hours to few days (Jones et al. 2009). We therefore used an in situ 13C pulse labelling experiment to follow these rapid allocation processes. A single cultivar was compared with catch crop mixtures of different diversity levels (4 and 12 species). We hypothesized that diversified catch cropping systems would (I) increase photosynthetic C fixation with higher transport rates of newly assimilated C to the roots, (II) stimulate microbial growth through higher C utilization when C reaches the rhizosphere and (III) prolongate the C cycling throughout the plant-soil-microbiome system.