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Competition for nutrients and water between crops and associated hedgerows reduces overall performance of contour hedgerow systems and hampers its acceptance by rural communities in tropical mountainous regions. Therefore, it is imperative to better understand competition leading to a decline in crop response close to hedges. In the highlands of North East Thailand spatial variability in grain yield of maize ( Zea mays L., cv. Suwan 1) was assessed for two contour hedgerow systems based on Brachiaria ruziziensis Germain et Evrard (Ruzi grass) barriers or Leucaena leucocephala (Lam) de Wit hedges without or with fertilizer (60 kg N ha−1 and 14 kg P ha−1). Available $$NO^{ - }_{3} - N$$ was analyzed across the slope. In addition, shoot N concentration and δ13C values in leaves were measured for maize plants in the center of the alley and in the row next to and at the upper side of barriers or hedges. Despite variable field conditions, δ13C values were significantly ( p < 0.05) less depleted close to the barriers or hedges, except for 2 out of 16 plots, suggesting that water deficiency was not the main driver for spatial variability along the alleys. The negative correlation between 13C isotopic discrimination and available $$NO^{ - }_{3} - N$$ in the soil, with R 2 ranging from 0.5 ( p < 0.10) to 0.9 ( p < 0.01), assigned a major role to N availability in the reduced crop response towards the barriers. The proposed framework of 13C isotopic discrimination, together with plant and soil N data, is a new approach and was shown to be suitable to determine N and water competition between hedgerows and crops grown in alleys under field conditions. [ABSTRACT FROM AUTHOR]

Cassava is the most important crop in DRC, where it is both a staple and a cash crop. Over the last years, cassava productivity seems increasingly constrained by climate disturbances. To respond to these climatic disturbances, farmers opt to adjust the planting period to spread the risk by optimizing their chances to receive the most regular rainfall in the crucial first 100 days after planting. To investigate the potential of varying cassava planting time to mitigate the effect of changes in rainfall patterns, a full-factorial field experiment was conducted in two contrasting sites in South Kivu (DRC) including Kalehe, a highland and forest region and Uvira, a dry savannah at medium altitude. Effects of three planting periods (November 2018, January 2019, and March 2019), with and without potassium nutrients (NP and NPK), were assessed on the growth parameters of two cassava cultivars, Obama (improved) and M′Bailo (landrace). The growth parameters included stem number, stem height, score of leaf retention and leaf lobe length and width in both sites up to four months after planting (MAP). Yield data on both the above-ground (stems and leaves) and the storage roots and related harvest index was only collected in Kalehe at 12 MAP. The highest storage root yield of 47 and 14 t ha−1 for Obama and M′Bailo respectively were achieved when the cultivars were planted in November 2018 and received NPK fertilizer. Compared to the early planted cassava (November 2018), cassava planted in January and March 2019 produced 29% and 79% fewer storage roots. Similarly, at the fourth month of each planting, growth parameters of early planted cassava were higher than those of late planted cassava although the later planted crop was exposed to more intensive and regular rains in the early stages. Hence the highest aboveground biomass was observed for Obama planted in November while the cassava planted in January and March produced 41% and 76% less above-ground biomass, respectively. Obama storage root yield represented on average 1.7 times M′Bailo storage root yield. K in the fertilizer amendment increased the storage root yield by 14% for M′Bailo and 20% for Obama as compared to no K fertilization. Obama had a better and more stable harvest index of 55% while the M′Bailo was 40%. Stem number, stem height, lobe length, and stem diameter were higher for both cultivars when planted in November 2018. This study revealed that the earlier the cassava was planted, the larger the yield. Thus, deciding to delay cassava planting to expose it to more regular rains in early stages can lead to an aggravation of the adverse effects of climate change. Although this study highlights the importance of combining early planting with improved cultivars for better yields, it did not establish evidence that the application of potassium could attenuate the negative effect of climate change on cassava productivity under rainfed conditions. • Climate change in DRC is expressed by disruptions in rainfall patterns at the end and the beginning of the cropping season. • Besides droughts, excessive rainfall at the beginning and end of cropping seasons is an overlooked agronomic challenge • Cassava planting time should consider rainfalls during the first 100 days and plant maturity at the start of dry season. • Cassava planting time should also consider the amount of rainfall following the onset of the rains after the long dry season. [ABSTRACT FROM AUTHOR]