Maize/faba bean intercropping with rhizobial inoculation in a reclaimed desert soil enhances productivity and symbiotic N₂ fixation and reduces apparent N losses

Highlights

• Rhizobium-inoculated faba bean/maize intercropping had advantages.

• The recovery of fertilizer N was increased by intercropping and rhizobium inoculation.

• Intercropping lowered accumulation of N_min in the soil profile at maize harvest.

• Intercropping with rhizobium inoculation enhances productivity and reduces the risk of environment.

Abstract

Although intensive modern agriculture, featured with high inputs of chemicals, contributed to global food production, simultaneously caused a series of serious issues, such as loss of biodiversity, soil degradation, and environment risk. The objective of our study is to integrate various inherit biological potentials of cropping system to gain higher productivity with lower external inputs. Field experiments were conducted over a two-year period (2008-2009) with five fertilizer-N application rates (0, 75, 150, 225 and 300 kg N ha⁻¹) to study the influence of fertilizer N application on the productivity and N utilization of a Rhizobium-inoculated maize (Zea mays L.)/faba bean (Vicia faba L.) intercropping system on a reclaimed desert soil (Sierozems). Average grain yields of faba bean and maize with inoculation increased by 50.0 and 19.6 %, respectively, and reached a maximum land equivalent ratio of 1.78 with zero fertilizer N application in 2009 compared with sole crops. The nodulation of intercropped faba bean was significantly higher with Rhizobium inoculation, especially with an optimum N application rate of < 150 kg N ha⁻¹. Average %Ndfa and Ndfa increased by 42.5 and 86.6 %, respectively, based on interspecific interaction and inoculation. The average apparent N recovery of the intercropping system with inoculation increased by 146.5 % at 75 kg N ha⁻¹ relative to the weighted mean values of sole crops. Intercropping saved fertilizer N compared to sole crops suggested by a fertilizer N equivalent ratio (NFER) > 1. Furthermore, intercropping with rhizobium inoculation saved more fertilizer N than that without rhizobium inoculation. After the faba bean harvest, intercropped maize made better use of the nitrogen taken up to grow faster than sole maize. At the maize harvest the intercropping system showed higher grain yield and N acquisition in the aboveground parts and lower Nmin accumulation in the soil profile, an effect that declined with fertilizer N application. The results therefore indicate that Rhizobium-inoculated maize/faba bean intercropping on the reclaimed desert soil with a moderate fertilizer N application rate enhancede productivity, biological N₂ fixation and apparent N recovery with lower environmental risk.

Introduction

Application of fertilizer N to agricultural soils is a useful strategy for achieving rapid growth responses for high crop productivity but the use of intercropping systems and inoculation with N₂-fixing rhizobia may be a preferred approach in the development of newly-reclaimed desert soils. This strategy may combine increased productivity with lower chemical N inputs through interspecific interactions and biological N₂ fixation. Newly-reclaimed soils in northwest China on the edge of the Mu Us desert are regarded as marginal lands because of poor soil structure, strong winds, heavy sedimentation, virtual drought conditions, and nutrient deficiencies. Nitrogen deficiency is one of the main characteristics of the newly-reclaimed soils. Chemical fertilizers including fertilizer N are convenient and rapidly available sources of nutrients for crop growth (Peoples et al., 1995). However, N is usually the most limiting nutrient in increasing crop productivity. The input efficiency of fertilizer N is low, seldom exceeding 50 %, and increases the environmental costs of N loss from fertilizers (Bohlool et al., 1992; Peoples et al., 1995). It is timely to reclaim desert soils using more sustainable methods based on the rational exploitation of natural resources with minimum negative impacts on the environment.

Legume/cereal intercropping is widely practiced as a sustainable food production system that reduces dependence on industrial N inputs by increasing symbiotic N₂ fixation (Adu-Gyamfi et al., 2007; Corre-Hellou et al., 2006; Hauggaard-Nielsen et al., 2003; Peoples et al., 2002; Neumann et al., 2007). Kessel and Hartley (2000) summarized the potential benefits of intercropping grain legumes with non-N₂-fixing crops as follows; (1) increasing total yield and increasing the land use efficiency (Mead and Willey, 1980), (2) reducing the need for fertilizer N either by increasing soil available N or by N transfer (Vandermeer, 1989), (3) increasing the utilization efficiency of available nutrients and water (Trenbath, 1976; Willey, 1979a, b), (4) diversification of the risks associated with crop failure or market instability (Fanning, 2000), and (5) reducing pest problems (Zhu et al., 2000). Intercropping is widely used in China as a system enhancing productivity (Li et al., 2013). Total biomass and grain yields of intercropped maize and faba bean were significantly higher than those of maize and faba bean in the corresponding sole crops on a calcareous soil in a field study by Li et al. (1999). Moreover, intercropping legumes with non-legumes increases the opportunity for N-use complementarity (Kessel and Hartley, 2000). As reported by Jensen (1996), intercropping pea and barley with or without a low rate of fertilizer N supply offers an opportunity to increase the input of symbiotically fixed N₂ into temperate agricultural ecosystems without compromising yield level and stability. In maize/faba bean intercropping systems there is substantial facilitation of nodulation and symbiotic N₂ fixation of faba bean where N derived from the atmosphere (Ndfa) by intercropped faba bean is > 98 % higher than that of monocropped faba bean (Fan et al., 2006). Similarly, peanut (Arachis hypogaea)/rice (Oryza sativa) intercropping increases N₂ fixation of intercropped peanut by 20 % compared to monocropped peanut (Chu et al., 2004). Similar interspecific facilitation of nodulation of legumes has been observed in our previous work (Li et al., 2001a, 2009) and in some other legume/cereal intercropping systems (Graham, 1981; Boucher and Espinosa, 1982; Santalla et al., 2001). More importantly, inoculation of legumes shows clear benefits (Singleton et al., 1992) and inoculation of grain legumes with rhizobia may increase biological N₂ fixation and crop yields. Increases in grain yields of pigeon pea inoculated with effective rhizobia ranged from 19 to 68 % over uninoculated controls (Nambiar et al., 1988). Furthermore, a beneficial effect of Rhizobium-inoculated legumes intercropped with non-leguminous species is generally expected. There is a complex interaction among the intercropped crops, inoculation with rhizobia, and indigenous bacteria and the increase in faba bean production in intercropping might be related to modification of the rhizosphere bacterial community (Zhang et al., 2010) and may increase soil fertility (Tian et al., 2019). Our previous studies show that faba bean inoculated with Rhizobium enhanced the biomass of both faba bean and maize in intercropping (Fang et al., 2009). Vance (2001) recommended the development of intercropping schemes that foster efficient use of N and P and also the assessment of the factors limiting rhizobial interactions with plants with the goal of site-(region) specific inoculation for sustainable agriculture with environmental amelioration. However, there is little or no benefit from legumes without rhizobial inoculation in newly-reclaimed soils because the nodules present are small or inactive. It is therefore necessary to inoculate the legumes. Little is known about the advantages of legume/cereal intercropping systems with rhizobial inoculation in newly-reclaimed desert soils (Mei et al., 2012) despite earlier studies carried out in relatively fertile soils. Cardoso et al. (2007) investigated a maize/common bean intercropping system with inoculation in southeastern Brazil and found that the equivalent yield increased by 31 %. Furthermore, Rhizobium increased the yields by 11 % and intercropping reduced the maize grain yields by 17 % but the system was shown to be more economically viable. Bacilio et al. (2006) attempted to use infertile desert soils amended with compost and inoculated with the bacterium Azospirillum brasilense. Arbuscular mycorrhizal inoculation of five tropical fodder crops was studied with organic matter in marginal soil (Gaur and Adholeya, 2002). Evidence from studies on nutrient-deficient soils indicates that diversity enhances agricultural productivity via rhizosphere P facilitation in P-deficient soils with faba bean/maize intercropping systems (Li et al., 2007).

Intercropping common bean with maize increases rhizobial efficiency, resource use efficiency and grain yield under low P availability (Latati et al., 2016). Aulakh et al. (2003) suggested that lower fertilizer P application rates can be used in an irrigated soybean–wheat rotation system on a subtropical semiarid soil. In addition, our contemporaneous study of maize/faba bean intercropping with rhizobial inoculation found enhanced productivity and recovery of fertilizer P in a P-deficient reclaimed desert soil (Mei et al., 2012). So far, little effort has been made to explore the potential of maize/faba bean-Rhizobium intercropping systems involving maize/faba bean and faba bean inoculated with suitable rhizobia, especially in newly-reclaimed soils, as a contribution to global food security.

Here, we report field experiments conducted over a two-year period to assess the effects of different application rates of fertilizer N on the yield advantage, plant yield, biological N₂ fixation and N utilization of a maize/faba bean-Rhizobium intercropping system and to evaluate whether the intercropping system with inoculation may be used as a sustainable strategy to increase productivity and minimize fertilizer inputs and apparent N losses in the long term.