Legume-grass mixtures improve biological nitrogen fixation and nitrogen transfer by promoting nodulation and altering root conformation in different ecological regions of the Qinghai-Tibet Plateau

Abstract

Introduction: Biological nitrogen fixation (BNF) plays a crucial role in nitrogen utilization in agroecosystems. Functional characteristics of plants (grasses vs. legumes) affect BNF. However, little is still known about how ecological zones and cropping patterns affect legume nitrogen fixation. This study's objective was to assess the effects of different cropping systems on aboveground dry matter, interspecific relationships, nodulation characteristics, root conformation, soil physicochemistry, BNF, and nitrogen transfer in three ecological zones and determine the main factors affecting nitrogen derived from the atmosphere (Ndfa) and nitrogen transferred (Ntransfer).

Methods: The ¹⁵N labeling method was applied. Oats (Avena sativa L.), forage peas (Pisum sativum L.), common vetch (Vicia sativa L.), and fava beans (Vicia faba L.) were grown in monocultures and mixtures (YS: oats and forage peas; YJ: oats and common vetch; YC: oats and fava beans) in three ecological regions (HZ: Huangshui Valley; GN: Sanjiangyuan District; MY: Qilian Mountains Basin) in a split-plot design.

Results: The results showed that mixing significantly promoted legume nodulation, optimized the configuration of the root system, increased aboveground dry matter, and enhanced nitrogen fixation in different ecological regions. The percentage of nitrogen derived from the atmosphere (%Ndfa) and percentage of nitrogen transferred (%Ntransfer) of legumes grown with different legume types and in different ecological zones were significantly different, but mixed cropping significantly increased the %Ndfa of the legumes. Factors affecting Ndfa included the cropping pattern, the ecological zone (R), the root nodule number, pH, ammonium-nitrogen, nitrate-nitrogen, microbial nitrogen mass (MBN), plant nitrogen content (N%), and aboveground dry biomass. Factors affecting Ntransfer included R, temperature, altitude, root surface area, nitrogen-fixing enzyme activity, organic matter, total soil nitrogen, MBN, and N%.

Discussion: We concluded that mixed cropping is beneficial for BNF and that mixed cropping of legumes is a sustainable and effective forage management practice on the Tibetan Plateau.

Keywords: cropping pattern; ecoregion; legume forage; rhizoma; root system; soil physicochemical; symbiotic nitrogen fixation.

Figure 1

Study area and cropping map.

Figure 2

Nitrogen content and accumulation in aboveground dry matter of oats and legumes in the different ecological zones under different cropping patterns. (A) Nitrogen content of aboveground dry matter in oats. (B) Nitrogen accumulation of aboveground dry matter in oats. (C) Nitrogen content of aboveground dry matter in pulses. (D) Nitrogen accumulation of aboveground dry matter in pulses. (E) Nitrogen content of aboveground dry matter in cropping systems. (F) Nitrogen accumulation of aboveground dry matter in cropping systems.

Figure 3

Characteristics of legume root tumors under different cropping patterns in different ecological zones. (A) Root tumor number. (B) Root tumor fresh weight. (C) Nitrogen-fixing enzyme activity.

Figure 4

Root system characteristics of monoculture and mixed cropping of oat and legume forages in different ecological zones. (A–D) Root dry weight. (E–H) Total root length. (I–L) Root area. (M–P) Root volume (Q–T) and root diameter. Lowercase letters indicate significant differences in the same ecological regions for different planting patterns, whereas uppercase letters represent significant differences in the same planting patterns for different ecological regions (p< 0.05). * denotes a significant difference between legume monoculture and mixture in the same ecoregion, ∗ denotes 0.01< p< 0.05, ∗∗ denotes p< 0.01, and ∗∗∗ denotes p< 0.001.

Figure 5

Soil physicochemical properties of the cropping patterns in different ecological regions. (A) Soil pH. (B) Soil organic matter. (C) Soil total nitrogen. (D) Soil ammonium nitrogen. (E) Soil nitrate nitrogen. (F) Soil microbial nitrogen.

Figure 6

Biological nitrogen fixation characteristics of legume forage under the different cropping patterns in different ecological zones. (A) Biological nitrogen fixation rate. (B) Biological nitrogen fixation. (C) Biological nitrogen fixation contribution.

Figure 7

Nitrogen transfer from legume forage to oats under different cropping patterns in different ecological zones. (A) Nitrogen transfer rate. (B) Nitrogen transfer. ∗ denotes 0.01< p< 0.05, ∗∗ denotes p< 0.01, ∗∗∗ denotes p< 0.001, and ns denotes p> 0.05.

Figure 8

Correlation between biological nitrogen fixation, nitrogen transfer, and biological nitrogen fixation contribution with cropping system (P), ecological region (R), climatic factors, root tumor characteristics, soil physical properties, and aboveground dry biomass. T, temperature; ASL, altitude; RW, root dry weight; RL, root length; RS, root surface area; RV, heel volume; RD, root diameter; NRN, number of rhizomes; NW, fresh weight of rhizomes; NG, nitrogen-fixing enzyme activity; DM, aboveground dry biomass. The width of the Mantel edge corresponds to the Mantel r value, and the color of the edge indicates statistical significance. ∗ denotes 0.01< p< 0.05, ∗∗ denotes p< 0.01.

Figure 9

Piecewise structural equation modeling (SEM) describing the effects of the cropping system in different ecoregions on biological nitrogen fixation and nitrogen transfer in legume forage. Effect of cropping pattern (R) and ecological region (P), nitrogen-fixing enzyme activity (NG), microbial mass nitrogen (MBN), nitrate nitrogen (NN), pH (Ph), total nitrogen (TN), nitrogen content (%N), and root surface area (RS) on biological nitrogen fixation (Ndfa) and nitrogen transfer (Ntransfer). Solid lines indicate significant effects, and dashed lines indicate non-significant effects. * and *** indicate significant differences at 0.05 and 0.001 levels, respectively.