Studies of stable isotopes of water in the environment have been fundamental to advancing our understanding of how water moves through the soil–plant–atmosphere continuum; however, much of this research focuses on how water isotopes vary in time, rather than in space. We examined the spatial variation in the δ18O and δ2H of throughfall and bulk soil water, as well as branch xylem and bulk leaf water of Picea abies (Norway spruce) and Fagus sylvatica (beech), in a 1‐ha forest plot in the northern Alps of Switzerland. Means and ranges of water isotope ratios varied considerably among throughfall, soil, and xylem samples. Soil water isotope ratios were often poorly explained by soil characteristics and often not predictable from proximal samples. Branch xylem water isotope values varied less than either soil water or bulk leaf water. The isotopic range observed within an individual tree crown was often similar to that observed among different crowns. As a result of the heterogeneity in isotope ratios, inferences about the depth of plant root water uptake drawn from a two end‐member mixing model were highly sensitive to the soil sampling location. Our results clearly demonstrate that studies using water isotopes to infer root water uptake must explicitly consider how to characterize soil water, incorporating measures of both vertical and lateral variations. By accounting for this spatial variation and the processes that shape it, we can improve the application of water isotopes to studies of plant ecophysiology, ecohydrology, soil hydrology, and palaeoclimatology.

No abstract

Intensive agriculture has major negative impacts on ecosystem diversity and functioning, including that of soils. The associated reduction of soil biodiversity and essential soil functions, such as nutrient cycling, can restrict plant growth and crop yield. By increasing plant diversity in agricultural systems, intercropping could be a promising way to foster soil microbial diversity and functioning. However, plant–microbe interactions and the extent to which they influence crop yield under field conditions are still poorly understood. In this study, we performed an extensive intercropping experiment using eight crop species and 40 different crop mixtures to investigate how crop diversity affects soil microbial diversity and activity, and whether these changes subsequently affect crop yield. Experiments were carried out in mesocosms under natural conditions in Switzerland and in Spain, two countries with drastically different soils and climate, and our crop communities included either one, two or four species. We sampled and sequenced soil microbial DNA to assess soil microbial diversity, and measured soil basal respiration as a proxy for soil activity. Results indicate that in Switzerland, increasing crop diversity led to shifts in soil microbial community composition, and in particular to an increase of several plant-growth promoting microbes, such as members of the bacterial phylum Actinobacteria. These shifts in community composition subsequently led to a 15 and 35% increase in crop yield in 2 and 4-species mixtures, respectively. This suggests that the positive effects of crop diversity on crop productivity can partially be explained by changes in soil microbial composition. However, the effects of crop diversity on soil microbes were relatively small compared to the effects of abiotic factors such as fertilization (three times larger) or soil moisture (three times larger). Furthermore, these processes were context-dependent: in Spain, where resources were limited, soil microbial communities did not respond to crop diversity, and their effect on crop yield was less strong. This research highlights the potential beneficial role of soil microbial communities in intercropping systems, while also reflecting on the relative importance of crop diversity compared to abiotic drivers of microbiomes and emphasizing the context-dependence of crop–microbe relationships.

Resource allocation to reproduction is a critical trait for plant fitness 1 , 2 . This trait, called harvest index in the agricultural context 3 – 5 , determines how plant biomass is converted to seed yield and consequently financial revenue of numerous major staple crops. While plant diversity has been demonstrated to increase plant biomass 6 – 8 , plant diversity effects on seed yield of crops are ambiguous 9 and dependent on the production syndrome 10 . This discrepancy might be explained through changes in the proportion of resources invested into reproduction in response to changes in plant diversity, namely through changes of species interactions and microenvironmental conditions 11 – 14 . Here we show that increasing crop plant diversity from monocultures over 2- to 4-species mixtures increased annual primary productivity, resulting in overall higher plant biomass and, to a lesser extent, higher seed yield in mixtures compared with monocultures. The difference between the two responses to diversity was due to a reduced harvest index of the eight tested crop species in mixtures, possibly because their common cultivars have been bred for maximum performance in monoculture. While crop diversification provides a sustainable measure of agricultural intensification 15 , the use of currently available cultivars may compromise larger gains in seed yield. We therefore advocate regional breeding programs for crop varieties to be used in mixtures that should exploit complementarity 16 among crop species.

By capitalising on positive biodiversity-productivity relationships, intercropping provides opportunities to improve agricultural sustainability. Intercropping is generally implemented using commercial seeds that were bred for maximal productivity in monocultures, thereby ignoring the ability of plants to adapt over generations to the surrounding neighbourhood, notably through increased complementarity, i.e. reduced competition or increased facilitation. This is why using monoculture-adapted seeds for intercropping might limit the benefits of crop diversity on yield. However, the adaptation potential of crops and the corresponding changes in complementarity have not been explored in annual crop systems. Here we show that plant-plant interactions among annual crops shifted towards reduced competition and/or increased facilitation when the plants were growing in the same community type as their parents did in the previous two generations. Total yield did not respond to this common coexistence history, but in fertilized conditions, we observed increased overyielding in mixtures with a common coexistence history. Surprisingly, we observed character convergence between species sharing the same coexistence history for two generations, in monocultures but also in mixtures: the six crop species tested converged towards taller phenotypes with lower leaf dry matter content. This study provides the first empirical evidence for the potential of parental diversity affecting plant-plant interactions, species complementarity and therefore potentially ecosystem functioning of the following generations in annual cropping systems. Although further studies are required to assess the context-dependence of these results, our findings may still have important implications for diversified agriculture as they illustrate the potential of targeted cultivars to increase complementarity of species in intercropping, which could be achieved through specific breeding for mixtures.