Effect of rock fragments content on water consumption, biomass and water-use efficiency of plants under different water conditions

Abstract

The proportion of rock fragments in soil affects water availability and therefore the characteristics of plants. The objective of this study was to evaluate the effect of rock-fragment content on plant water consumption, biomass, growth and water-use efficiency (WUE) under different water conditions. Four gravimetric treatments of rock-fragment contents (0, 10, 30 and 50%) and four treatments of water content were tested in sandy loamy soils. The water contents of the rock-free soil were 15–19% (80–100% of field capacity), 11–15% (60–80% of field capacity), 9–11% (47–60% of field capacity) and 6–9% (32–47% of field capacity). Transpiration, plant height, basal stem diameter and biomass of korshinsk peashrubs in the treatments were measured and compared. Plants grown in the soil with rock fragments transpired less, especially under well-watered conditions. The mean daily transpiration of plants in the soils with 30 and 50% rock-fragment contents was 18% (P = 0.021) and 34% (P = 0.001) lower, respectively, in 2014, and 25% (P = 0.008) and 31% (P = 0.002) lower, respectively, in 2015 relative to the soil without rock fragments and was not lower in the soil with 10% rock fragments. Plant height, basal stem diameter and biomass did not differ significantly between rock-fragment contents of 0 and 30% but were lower at 50%. WUE, the ratio between total transpiration and biomass, was highest at 30% and then decreased at 50%. Increasing plant water stress could thus improve WUE. The rock fragments in the soil had significant effects on plant water consumption, biomass, growth and WUE. Optimizing the rock-fragment content is necessary when the relationships between plants and water in stony ecosystems are evaluated.

Introduction

A soil rock fragment is defined as a particle with a diameter >2 mm. Fragments forming as a result of processes of soil genesis and human activity exist at the soil surface and within the soil. Stony soils are widespread and can reach land-cover ratios up to 60% in the Mediterranean region of Western Europe (Poesen and Lavee, 1994). The proportion of stony soil is >30% on the Loess Plateau of China (Hou, 1993).

The presence of rock fragments significantly affects hydrological functioning such as water storage, infiltration and evaporation and soil hydraulic properties such as hydraulic conductivity and water retention (Van Wesemael et al., 1996, Ma and Shao, 2008, Li et al., 2008, Zhou et al., 2009, Baetens et al., 2009, Ma et al., 2010, Novák et al., 2011, Tetegan et al., 2011). Water movement in, and the hydraulic properties of, soils are strongly dependent on the availability of water for plants. Rock fragments in stony areas should therefore be considered in studies of water availability for plants and of water exchange among plants, rock fragments and soils.

Tetegan et al. (2011) proposed pedotransfer functions based on the linear relationship between the available water content (AWC) of rock fragments and the Napierian logarithm of bulk density and the relationship between water content at −100 and −15 840 hPa. The simulation showed that excluding 30% of the pebbles in a stony horizon underestimated the soil available water content (SAWC) by 5% for chert pebbles and by 33% for chalk pebbles. Novák and Kňava (2012) demonstrated that the presence of stones can decrease soil water-holding capacity and hydraulic conductivity, which can decrease the availability of soil water for trees. Tetegan et al. (2015a) subsequently improved the method proposed in 2011 (Tetegan et al., 2011) to calculate the SAWC at a regional scale (36 200 ha). They demonstrated that rock fragments could contribute to the AWC of stony soils. SAWC could thus be underestimated if rock-fragment content is neglected. Water exchange would be more complex when plants are added to the system. Tetegan et al. (2015b) included the dynamics of water exchange between fine and stony soil. They demonstrated that rock fragments in soil could act as water reservoirs for plants. Water was exchanged between rock fragments and soils, plants and soils, and plants and rock fragments.

The growth and physiological features of plants should differ between stony and rock-free soils due to the effect of rock fragments on the water availability for plants and water exchange. Danalatos et al. (1995) reported that water conservation was generally better in stony soils under conditions of moderate water stress, and the presence of cobbles on the soil surface increased total dry-matter yield of rainfed wheat by 20%. The presence of stones in soil also affects the growth of plant roots. Estrada-Medina et al. (2013) demonstrated that rock fragments changed the distribution of root systems. Plant roots can grow in both soil and rock. Changes in plant growth have been attributed to hydraulic and nutrient redistribution resulting from the presence of rock fragments (Carrick et al., 2013). Root growth is linked to the compressive strength of rocks; the penetration resistance of soil ranges from 2 to 4 MPa (Arshad et al., 1996), and root growth is restricted above this range (Schwinning, 2013).

In addition to changing the yield and root distribution of plants, rock fragments can also change other features of plants such as transpiration (a feature of water consumption), stem height and diameter (features of plant growth), and water-use efficiency (WUE). Few studies, however, have investigated changes in these features, which limits our understanding of the features of plant growth in stony soil.

We hypothesized that (1) the responses of plant transpiration, stem height and diameter, and biomass to water conditions can be affected by the rock-fragment content of the soil and (2) rock-fragment content would affect WUE under different water conditions. The objectives of this study were therefore to (i) assess whether the features of plants such as transpiration, stem height and diameter, and biomass respond to rock-fragment content under different water conditions, and (ii) quantify and compare WUEs and then determine a feasible strategy of management of rock fragments and water conditions for increasing WUE.