Monoculture vs intercropping under drought: functioning of microorganisms and the fate of maize and soybean 13C-labeled rhizodepositions

Drought due to climate change will be more frequent and severe and expected to cause a loss of agricultural productivity in the next 30 years. Drought affects soil water availability, leading to several effects: reduction of crop growth, increases shoot/root ratio, decreases the uptake of nutrients by roots and translocation from roots to shoots, and availability of nutrients (N, P, S, Ca, Mg) in soil. The plant community regulates the composition and quantity of rhizodeposition, however, due to the impact of drought on plants functioning, allocation and fate of assimilated carbon (C) in the plant-soil-microorganisms system is changed.  Especially strong effects of drought on the functioning of soil microbial communities are seen in the crop monocultures, whereas the introduction of intercropping, especially with legumes, can potentially mitigate the drought impact. Thus, the present study aimed i) to trace the fate of rhizodeposits in the maize-soybean intercropping and compare with that in monoculture under drought vs optimal moisture conditions, ii) to reveal the differences in the rhizodeposits-C utilization by microorganisms under pulse water regime (wet to drought vs drought to rewetting).

The greenhouse experiment was conducted in June-August 2021. The soils were taken from the 10-old abandoned agricultural land in Witzenhausen, sieved through 4 mm, and 3 kg were placed into the incubation pots (15x15x20 cm). Maize (Zea mays L.) and soybean (Glycine max (L.) Merr.) were grown as monoculture (4 plants per pot) or intercropping (2+2 plants) until the vegetative stage (6-8 leaves), soil moisture was kept at 60% of WHC. Further, plants were divided into four treatments (each had four replicates), and each was labeled with K¹⁵NO₃ and followed by ¹³C-CO₂ (one day later). The treatments were: i) constant wet (60% of WHC), ii) constant drought (30% of WHC), iii) wet to drought (labeling was done under 60% of WHC and soil dried till 30% of WHC and further kept for 14d), and iv) drought to wet (labeling was done under 30% of WHC and soil was slowly rewetted to 60% of WHC and further kept for 14d). Subsamples of soils and plants aboveground biomass were done 4 times: i) 1 day after ¹³C-labeling, ii) after soil reached drought (30% of WHC) or 60% of WHC after rewetting, iii) in the middle of a drought or rewetting stage and iv) at the end of the experiment. Constant moisture and unlabelled treatments were sampled at the same time points; total and ¹³C/¹⁵N were analyzed in plant biomass, dissolved organic matter (DOM), microbial biomass, and soil; NO₃^- and NH₄⁺ contents were measured in DOM extracts; additionally, activities of C, N, and phosphorus acquisition enzymes were measured. Thus, results from this experiment will shed light on the fate of rhizodeposits depending on the drought conditions and pulse water regime. The comparison of monoculture with intercropping will reveal how the presence of soybean can improve the nutrition of maize regarding N uptake, and whether microorganisms can be less affected by drought under intercropping.