Nitrogen (N) serves as an essential nutrient for yield formation across diverse crop types.  However, agricultural production encounters numerous challenges, notably high N fertilizer rates coupled with low N use efficiency and serious environmental pollution.  Deep placement of nitrogen fertilizer (DPNF) is an agronomic measure that shows promise in addressing these issues.  This review aims to offer a comprehensive understanding of DPNF, beginning with a succinct overview of its development and methodologies for implementation.  Subsequently, the optimal fertilization depth and influencing factors for different crops are analyzed and discussed.  Additionally, it investigates the regulation and mechanism underlying the DPNF on crop development, yield, N use efficiency and greenhouse gas emissions.  Finally, the review delineates the limitations and challenges of this technology and provides suggestions for its improvement and application.  This review provides valuable insight and reference for the promotion and adoption of DPNF in agricultural practice.

Drought is an important abiotic stress factor in cotton production.  The root system architecture (RSA) of cotton shows high plasticity which can alleviate drought-related stress under drought stress (DS) conditions; however, this alleviation is cultivar dependent.  Therefore, this study estimated the genetic variability of RSA in cotton under DS.  Using the paper-based growth system, we assessed the RSA variability in 80 cotton cultivars at the seedling stage, with 0 and 10% polyethylene glycol 6000 (PEG6000) as the control (CK) and DS treatment, respectively.  An analysis of 23 above-ground and root traits in the 80 cotton cultivars revealed different responses to DS.  On the 10th day after DS treatment, the degree of variation in the RSA traits under DS (5–55%) was greater than that of CK (5–49%).  The 80 cultivars were divided into drought-tolerant cultivars (group 1), intermediate drought-tolerant cultivars (group 2), and drought-sensitive cultivars (group 3) based on their comprehensive evaluation values of drought resistance.  Under DS, the root length-lower, root area-lower, root volume-lower, and root length density-lower were significantly reduced by 63, 71, 76, and 4% in the drought-sensitive cultivars compared to CK.  Notably, the drought-tolerant cultivars maintained their root length-lower, root area-lower, root volume-lower, and root length density–lower attributes.  Compared to CK, the root diameter (0–2 mm)-lower increased by 21% in group 1 but decreased by 3 and 64% in groups 2 and 3, respectively, under DS.  Additionally, the drought-tolerant cultivars displayed a plastic response under DS that was characterized by an increase in the root-lower characteristics.  Drought resistance was positively correlated with the root area-lower and root length density-lower.  Overall, the RSA of the different cotton cultivars varied greatly under DS.  Therefore, important root traits, such as the root-lower traits, provide great insights for exploring whether drought-tolerant cotton cultivars can effectively withstand adverse environments.

The exogenous application of melatonin by the root drenching method is an effective way to improve crop drought resistance.  However, the optimal concentration of melatonin by root drenching and the physiological mechanisms underlying melatonin-induced drought tolerance in cotton (Gossypium hirsutum L.) roots remain elusive.  This study determined the optimal concentration of melatonin by root drenching and explored the protective effects of melatonin on cotton roots.  The results showed that 50 μmol L^–1 melatonin was optimal and significantly mitigated the inhibitory effect of drought on cotton seedling growth.  Exogenous melatonin promoted root development in drought-stressed cotton plants by remarkably increasing the root length, projected area, surface area, volume, diameter, and biomass.  Melatonin also mitigated the drought-weakened photosynthetic capacity of cotton and regulated the endogenous hormone contents by regulating the relative expression levels of hormone-synthesis genes under drought stress.  Melatonin-treated cotton seedlings maintained optimal enzymatic and non-enzymatic antioxidant capacities, and produced relatively lower levels of reactive oxygen species and malondialdehyde, thus reducing the drought stress damage to cotton roots (such as mitochondrial damage).  Moreover, melatonin alleviated the yield and fiber length declines caused by drought stress.  Taken together, these findings show that root drenching with exogenous melatonin increases the cotton yield by enhancing root development and reducing the root damage induced by drought stress.  In summary, these results provide a foundation for the application of melatonin in the field by the root drenching method.