Allometric Model for Predicting Root Biomass of Field Crops in the Salt-Affected Clay Soil: Novel Approach ^†

Abstract

Root biomass and phenotyping are vital parameters for studies on crop performance and response to environmental change, as well as abiotic stresses, crop water uptake, nutrient supply, and soil C sequestration and quality. However, root sampling and measurement, including biomass estimation, are laborious and time-consuming tasks. This study developed a novel allometric model to predict the root biomass of annual crop species using root collar diameter, an easy aboveground field measure. The root samples of alfalfa, sorghum and maize were collected (45 from each) at the harvesting stage from the irrigated agricultural field of the semi-arid region (clay soil, salinity: EC = 2–12 dS m⁻¹, 70% of full irrigation). Crops collar diameter (CD) and root biomass (RM) increased in the following order: alfalfa < sorghum < maize. For each crop species, strong power (RM = aCD^b) relations (R² ≥ 0.90) were found between RM and CD (analogous to tree species). The coefficient (a) and exponent (b) of the relations and the soil quality indices (e.g., soil organic carbon, aggregate stability) in the root zone were concomitant with the crop (root) traits. The use of the allometric model was crucial for the fast assessment of the root biomass of the crop species, such as estimating biomass allocation. The approach could be used for evaluation of soil–root–plant interaction under abiotic stresses in the context of the sustainable agriculture (e.g., soil C deposition and respiration, crop transpiration and photosynthesis rate, and selecting the best genotypes-cultivars).

1. Introduction

Crop root systems play a crucial role in crop performance and soil C cycle, and affect (i) soil quality parameters, water and nutrient availability; (ii) enzyme activity, microbial biomass and respiration and microbial community structure; and (iii) soil aggregate stability, water retention and hydraulic conductivity and resistance against erosion. Soil organic carbon (SOC) accumulation is related to the interactions of several ecosystem processes (respiration, decomposition, and photosynthesis), yet C sequestration is largely mediated by crops via photosynthesis. The input rate of SOC is governed mostly by the crop root biomass and litter, and indirectly by the transfer of C-enhanced mixtures from roots to soil microbes. The fixation of atmospheric CO₂ into crop biomass occurred by photosynthesis; distribution of photosynthate to roots, stems, and leaves are affected by the environmental factors and ontogenetic drifts. The roots depend on the shoot for carbohydrates, whereas the shoot relies on the root for water and nutrients. In arid and semi-arid regions, environmental abiotic stresses (e.g., drought, salinity, temperature) increase the relative weights of root biomass and its phenotyping. Thus, incorporating the easily assessable parameters of the root system into soil-crop model components will be critical for evaluating the use of the available water and nutrient resources, and climatic change impact on crop productivity and soil quality [1,2,3,4,5,6].

To quantify crop roots biomass is difficult, since roots are challenging to measure in the soil, and sampling is laborious, especially extraction from clay soil, and is time-consuming [7]. The allometric relations between plant organs (shoot, root or leaf biomass, crop height and diameter, leaf area index, root length) link plant structural development and physiological processes, and are widely used to study the fundamental mechanisms of plant function. In trees, root biomass can be projected indirectly by using allometric relations, such as a root-to-shoot ratio, and by linking the root and shoot biomass with plant height and trynk diameter. It is well established that tree root and shoot biomass can be predicated from the stem or collar diameter [8,9,10]. Such methods could be useful for agricultural annual crops as well, yet regardless of the approach used, it is necessary to measure the root system directly. The ability of crops to adapt to the environment and abiotic stress conditions affect crop organ mass fraction in both trees and annual crops, and become evident in the parameters of allometric relations [8,9,10,11,12,13,14]. Therefore, the objective of the study was to develop an allometric model to predict the root biomass of crop species using a root collar diameter, one of the most easily measured crop parameters.

2. Material and Methods

An experiment was conducted on the semi-arid irrigated and artificially drained field, located in the middle part of the Kur-Aras lowland region of Azerbaijan. Soil was a swelling and crusting clay. Due to the natural condition and farming history, soil salinity distribution was heterogeneous (EC = 2–12 dS m⁻¹) in the upper cultivated layer, and increased considerably with soil depth. Mineralized ground water (4–16 g L⁻¹) was situated 1.2–1.8 m from the soil surface. Three crop species, varying in root system architecture (alfalfa, Medicago sativa; sorghum, Sorghum bicolor; maize, Zea mays), were grown in large plots (40 × 40 m; 50 × 20 spacing) through April–August, over three years. Irrigation rate was traditional (70% of full irrigation frequently applied by farmers) due to the fresh water shortage. Root samples were collected (3 × 45 = 135 samples, 0–40 cm depth) using a monolith excavation method at the harvesting stage [7,14].

The properties of soil samples from root zone were analyzed using standard methods. Root separation was completed carefully by hand and washing (elutriation system) over a sieve (≥0.5 mm); its dry biomass were established by forced-air oven drying at 65 °C. Root collar diameter (CD) was measured by the digital caliper. An ANOVA was conducted to compare the role of crop species on vegetative parameters, and mean comparisons were made by the Tukey HSD (p < 0.05). Least squares fitting was used to evaluate the allometric relation (linear, exponential or power) between the RM and the CD [9,10,14].

3. Results and Discussions

The crop species produced varied root biomass in swelling clay soil, associated with salinity and water stresses. Crops RM mostly was clustered within the upper 30 cm of soil layer (>90%). Both the mean of RM (e.g., mean ± se: maize: 12.28 ± 1.36; sorghum: 7.92 ± 0.61, alfalfa 0.92 ± 0.11 g) and CD (13.56 ± 0.65; 11.82 ± 0.38; 3.54 ± 0.25 mm) increased in the following order of crops: control (no crop) < alfalfa < amaranth < maize. There were strong power relations (RM = aCD^b; R² ≥ 0.90, p < 0.05) between RM and CD for each crop species, as well as power or exponential (not given) relation (R² = 0.93) for all merged crop species (Figure 1). The differences between the coefficients (a = 0.031, 0.033, 0.105) and exponents (b = 2.22, 2.19, 1.61) of the allometric model was anticipated, since crops with different root types (e.g., tap, fibrous, adventitious; coarse or fine), and widely varying in RM were used. The generalized power or exponential model that merges three crops (Figure 1) may predict the root biomass with reliable accuracy (R² = 0.93) for comparable soil and environmental condition, particularly when obtaining such data is challenging, and approximate estimation is acceptable for the desired strategy. The study is continued with six annual crop species and a better generalized model is expected.

The largely recognized relations between the height, biomass and CD of the trees could also be used for herbaceous plants with small vertical stems, including used annual field crops [8,10,12,13,14]. Therefore, if the effect of plant genotype on RM is allocable under various environmental conditions, the allometric models can reflect water and salinity (and nutrient) stress, plant tolerance level, and the input of soil quality [8,10,14]. In the studied region, the root to shoot ratio of annual field crops (0.1–0.3) is mostly related to the abiotic stresses associated with the elevated salinity and scarcity of irrigation water. The order of the RM was somewhat opposite to the root to shoot ratio, showing that sorghum or maize were crops more resistant to the combined abiotic stresses than alfalfa [14].

The cropping improved SOC content and aggregate stability that was significantly higher under crops than in the control (Figure 2). Soil aggregation and pore size distribution in the root zone was frequently modified during the period of active root growth, and by the microbial activity. Roots rhizodeposit (dead fine roots, exudates, mucilages, secretions of insoluble materials, and lysates) differ with crop species, and play a vital role in ecosystem functions; it also form C sources, and enhances SOC and soil structure. However, RM correlated unfairly with SOC and structure stability (Figure 2), which could be explained by the variation in root system attributes, and the rhizodeposition of crop species, and soil texture, since high aggregate stability of clay soil may surpass the role of crops RM [1,2,3,4,5,6]. Coupled salinity and water stress, and increase in salinity with depth, combined with clay soil texture stress, likely (i) affected the fraction of root system modules, and rooting depth, and water use efficiency triggered by a saline gradient and moisture content; (ii) increased crop stomatal resistance under deficit irrigations, and decreased respiration, transpiration and photosynthetic capacity of crops; and (iii) consequently reduced root and shoot biomass, although roots were less affected by salinity [11,14].

4. Conclusions

Roots play a crucial role in the crop performance and C sequestration, yet selection of the cultivars or crop functional traits are mainly based on shoot-related phenotypes. This study provides a new allometric model for predicting RM from measurements of CD; the relation suggests that various effects of stresses will be laid in the line of the curve. To our knowledge, the study provides the first novel model predictions against direct RM data obtained from field measurements. The RM can be linked to the available data on shoot biomass and crop height (similar to the allometric relations widely used for forest trees) for selecting and breeding of the crop, and functional traits to maximize water and nutrient use efficiency with a consideration of management and climate change scenarios [1,4]. It can be applied to the cropping system for sustainable management, and soil quality improvement associated with C sequestration and enhanced soil processes. Use of this approach in the semi-arid Kur-Araz lowland or other regions with a similar soil type, irrigation requirements, and environmental conditions, seems promising.