Belowground nitrogen transfer from legumes to non-legumes under managed herbaceous cropping systems. A review

Abstract

Nitrogen is the most limiting nutrient in most agroecosystems and thus critical for sustaining high yields. Conventional agricultural practices use synthetic fertilizers to ensure an adequate supply of nitrogen in soils, but fertilizers come at a significant monetary and environmental cost. A strategy to improve nitrogen supply in cropping systems is the inclusion of nitrogen-fixing legumes, which can provide nitrogen benefits to companion crops through belowground nitrogen transfer. However, a better understanding of the underlying mechanisms and factors that govern nitrogen transfer is important in order to determine potential areas for improving this association. Here, we review the mechanisms of belowground nitrogen transfer in managed herbaceous cropping systems, focusing on forage systems. We classify three major routes of nitrogen transfer from legumes to non-legumes: (1) decomposition of legume root tissues and uptake of mineralized nitrogen by neighboring plants, (2) exudation of soluble nitrogen compounds by legumes and uptake by non-legumes, and (3) transfer of nitrogen mediated by plant-associated mycorrhizae. Literature data shows that rates of nitrogen transfer range from 0 to 73 % from forage legumes to companion grasses in mixed stands, depending on the legume species and cultivar. We list the factors that affect nitrogen transfer including abiotic factors, e.g., water stress, temperature, light, soil available nitrogen, and application of nitrogen fertilizer, and biotic factors, e.g., root contact, plant density, growth stage, production year, defoliation, and root herbivores. While the rates of nitrogen transfer are often constrained by abiotic conditions, such as temperature and water availability, that are beyond the control of growers, agronomic practices, e.g., planting density and choice of species and cultivar, may help to increase nitrogen transfer. Ultimately, the selection of plant pairs with compatible traits offers the best path forward to improving nitrogen transfer in intercrops.

Challenges in Using Precision Agriculture to Optimize Symbiotic Nitrogen Fixation in Legumes: Progress, Limitations, and Future Improvements Needed in Diagnostic Testing

“…N fixed by legumes can also be released from root systems during the growing season (mainly as root exudates) [8,114] or during the subsequent growing season (primarily due to decomposition) [7]. Organic N release helps to improve soil N availability for subsequent crops and promotes N transfer to neighboring non-legumes in a mixed/intercropped field.…”

“…As a result, legumes are rich sources of high quality protein for humans (e.g., lentil, beans, peanut) and livestock (e.g., alfalfa, clover, vetch) [5,6]. Legume crops can also enrich the soil N through organic N deposition in various cropping systems and crop rotations, when used as green manures or when a perennial legume is used [7][8][9]. However, legume crops growing in many agroecosystems suffer from suboptimal SNF, due to suboptimal biological factors (e.g., unavailability of compatible soil rhizobia or competition by indigenous rhizobia) and environmental factors (e.g., inadequate micronutrients, drought, high/low temperature, salinity, and pH).…”

Nitrogen Fixation and Transfer in Agricultural Production Systems

“…In fact, N is the third most important factor in the growth and development of crop plants [1]. This made N one of the most important nutrients in agricultural production systems.…”

“…Among the issues of great importance is the transfer of fixed N 2 to non-N 2 -fixing plants. The proportion of biologically fixed N 2 transferred to neighboring plants can range from as low as 0% to as high as 73%, depending on a myriad of factors [1]. The biology, chemistry, and processes involved in BNF have been extensively described in the literature [7][8][9][10][11][12].…”

Efeito do cultivo intercalar de adubo verde com minitomateiro orgânico em ambiente protegido na produtividade, qualidade e transferência de nitrogênio

“…Além disso, as raízes também podem ser responsáveis pela transferência de N através de exsudados radiculares a outras plantas, principalmente quando há contato entre as raízes (Lesuffleur et al, 2013;Zang et al, 2015;Thilakarathna, Mcelroy, Chapagain, Papadopoulos, & Raizada, 2016). Desse modo, há uma possibilidade de que das raízes dos adubos verdes, que ocupam grande volume de solo (Tardy et al, 2017), terem se conectado às plantas de minitomate que estavam ao seu arredor, inclusive as testemunhas.…”