Ecosystem functioning of arable land in response to cropping systems and drought

Abstract

Conflicts between increasing food demands and adverse effects of agricultural intensification on the environment, together with frequent and severe drought, are threatening global food production and food security. Therefore, agricultural systems have to be more productive, sustainable, and resilient in the context of global change. Organic farming and conservation tillage have been widely implemented as means of ecological intensification due to their ecological benefits, such as enhancing biodiversity, reducing greenhouse gas emissions, and improving soil fertility. However, how these cropping systems can potentially compensate drought effects on food production and functioning of ecosystems was scarcely reported in arable lands. Therefore, as introduced in Chapter 1, this doctoral thesis focuses on the capability of different cropping systems to mitigate drought effects on ecosystem functioning, including crop yields and total N uptake, crop phenology, plant water uptake, litter decomposition, and soil nitrate availability. As introduced in Chapter 2, all the experiments in this thesis were conducted using the FAST (Farming Systems and Tillage Experiment) trial, which compares organic farming vs. conventional farming with different tillage depth, resulting in a total of four cropping systems, i.e. i) organic farming with intensive tillage, ii) organic farming with reduced tillage, iii) conventional farming with intensive tillage, and iv) conventional farming with no-tillage. During major crop growth stages, summer drought treatments were applied with portable rain shelters for all four cropping systems. Crop phenology helps schedule management practices and acts as a biological indicator integrating the information from both environmental drivers and management options. However, the study on crop phenology as affected by smallholder management practices is often limited by the temporal and spatial resolution of phenological observations. Therefore, the goal of Chapter 3 was to investigate the feasibility of PhenoCams, i.e. time-lapse cameras, for tracking crop phenology and to assess how cropping systems affect crop phenology and estimating yields at harvest with phenology observations. During the years 2018 and 2019, we monitored vegetation changes among four cropping systems during two crop growing seasons, i.e. pea-barley mixture and winter wheat monoculture. The results indicated that early-season phenological differences established by cropping systems in winter wheat can be well translated into changes in crop yields and total N uptake under ambient rainfall conditions. The response of crops to drought can be very different depending on the crop species and growth stages. Timely observation of vegetation changes can monitor drought effects on crop development over time. In Chapter 4, the crop yields, total N uptake and phenology in response to drought among different cropping systems were investigated. The drought response of phenology was not consistent among crops, with up to 6 days earlier onset of several phenological metrics in pea-barley, but no such shifts of these metrics in winter wheat. Temporal interactions of systems with drought on phenology were shown for both pea-barley and winter wheat. Compare to control, drought treatments caused 20–27% and 16–21% reductions of grain yields and total N uptake for pea-barley mixture and winter wheat, respectively. The findings of this chapter indicate that cropping systems cannot compensate the negative effects of drought on phenology nor crop yields or total N uptake. Plants can shift their growth stages to escape from unfavorable conditions (drought escape mechanism), or avoid drought by developing deeper roots or utilizing water in a deeper soil layer (drought avoid mechanism), or can be both. To better understand the mechanisms of crops in response to drought, in Chapter 5, the soil depths of pea-barley water sources among all cropping systems combined with drought treatment were investigated using stable water isotopes. Pea plants prefer shallower soil water as compared to barley plants. As affected by drought, both pea and barley relied more on shallower soil water (0-20 cm) across all cropping systems than the control treatment, but cropping systems did not shift the soil water sources of both pea and barley. Thus, adapting cropping systems could not mitigate the drought effect on plant water use strategy. Drought can not only lead to phenological shifts and physiological failures on plant-level but also inhibit ecosystem processes and functions in soil. Litter decomposition releases nutrients from organic matter to the atmosphere, soil organisms and crops and can be primarily inhibited under drought. However, it is unclear to what extent organic farming and conservation tillage can alleviate drought effects on litter decomposition. Therefore, in Chapter 6, the effect of drought on litter decomposition was examined in four cropping systems across three crop growing seasons, including pea-barley mixture, maize monoculture, and winter wheat monoculture. Using two standard tea litters with distinct quality, i.e., high-quality green tea with a narrow C: N ratio and low-quality rooibos tea with a wide C: N ratio, it was possible to assess the effects of cropping systems on litter decomposition and especially on the resistance (i.e., the ability to withstand a disturbance) and the resilience (i.e., the ability to return to undisturbed conditions). Cropping systems had no effect on litter decomposition, regardless of litter quality and drought treatment. The decomposition of high-quality litter was less resistant but more resilient to drought, and vice versa for low-quality litter. Soil nitrate availability was also strongly decreased by drought (by 32 to 86%). The positive correlation of soil nitrate availability with litter decomposition was significant for pea-barley but not for winter wheat during drought, but this correlation disappeared upon rewetting. In summary, the results of this chapter reveal neither adaptation of organic farming nor conservation tillage can sustain early-stage litter decomposition in response to severe drought. As synthesized in Chapter 7, this doctoral thesis studied the resistance and resilience of ecosystem functions of different cropping systems to drought, in terms of crop productivity, crop phenology, plant water uptake pattern, litter decomposition and soil nitrate availability. The results suggest that stabilizing crop yields and ecosystem functioning in response to drought may not be possible when applying organic farming and conservation tillage for short terms (less than 10 years). For further study, as PhenoCams observe crop phenology in real time, also regarding phases that are not visible for human observers, it is possible to test various management options, such as cover crops or precise irrigation or fertilization on specific crop growth phases to improve current cropping systems.