The effect of natural volcanic tuff as a mulching on plant growth and soil chemistry was determined under different water stress levels (75%, 65%, 55%, and 45%). The effect of irrigation regimes and mulching was determined through many indicators, namely plant growth, plant water relation, leaf water potential, and soil chemistry.
For both types of soil (S1 and S2), water stress showed a high impact on the plant height (Fig. 1), number of branches (Fig. 2), trunk diameter (Fig. 3), shoot length (Fig. 4), shoot diameter, and number of leaves (Fig. 5) where the plant growth decreased significantly with water stress. In contrast, mulching with volcanic tuff provided significant improvement in plant growth for all water regimes. In comparison with S1W1 (control soil, 75% of field capacity), S1W4 (control soil, 45% of field capacity) showed a significant reduction in the number of branches, trunk diameter, shoot length, shoot diameter, number of leaves, and plant weight up to 32%, 27%, 38%, 33%, 48%, and 30%, respectively, indicating a high impact of water stress on the plant growth. In case of mulching soil with volcanic tuff, S2W4 showed a less reduction with 23, 18, 32, 25, 43, and 21% for the same parameters, respectively, in comparison with S1W4. This result revealed that mulching with volcanic tuff could be used to minimize the negative impact of draught and decrease the destructive effect of water stress. Regardless of irrigation level, plant height, number of branches, trunk diameter, shoots length, shoot diameter, number of leaves, and plant weight for soil covered with volcanic tuff were significantly greater than those of plants grown in silty clay soil without mulching after the fifth growing season. Mulching treatment increased number of leaves by 2.85%, 7.91%, 14.22%, and 14.5%, number of branches (8.2%, 16.9%, 14%, and 18.3%), plant height (0.3%, 0.4%, 0.9%, and 0.6%), plant weight (10.73%, 18.62%, 9.63%, and 13.7%), trunk diameter (9%, 9.4%, 13.9%, and 12.5%), shoot length (3%, 11.4%, 13.9%, and 14.8%), and shoot diameter (11.1%, 12.5%, 8.6%, and 12.5%) for W1, W2, W3, and W4, respectively. It can be concluded that for all irrigation regimes, mulching increased the morphological performance but its effects were more pronounced under high water stress.
Plant water relations
Relative water content (RWC) and leaf water potential are significant indicators of water status in plants in terms of the physiological consequence of cellular water deficit which reveals the equilibrium between water supply to the leaf tissue and transpiration rate. A relative water content in the leaves was much greater under higher irrigation (W1) and decreased significantly under severe water stress (24%) (Fig. 6). Mulching treatment had a significant effect on RWC, which surpassed that for without mulching by 14%. Among the different irrigation levels, relative water content increased slightly (9%) with mulching under adequate moisture content (S2W1) but increased considerably (20%) under deficient irrigation (S2W4). Also, this result buttressed the higher impact of volcanic mulching at higher water stress.
Leaf water potential
Leaf water potential was significantly affected by water availability (Fig. 6). Under control soil (S1), leaf water potential with sufficient irrigation (W1) was significantly greater than other treatments of irrigation and decreased rapidly by 19.4%, 40%, and 52% after subjecting the plants to water stress W2, W3, and W4, respectively. The leaf water potential of plants grown in mulched soil was significantly greater than those of untreated soil by 19.3% (W1) to 32.7% (W4) with an average value of 16%.
Effect of tuff mulching on soil minerals
At the end of the investigational period, in December 2016, total cations, CaCO3, pH, Na, Mg, Ca, SAR, ESP, OM, EC, N, and K contents of the soil were determined. Total cations, Na, Ca, ESP, EC, N, K and SAR contents increased in soil covered with volcanic tuff significantly compared to control one by 22%, 39%, 24%, 48%, 25%, 43%, 18 and 33%, respectively (Fig. 7). In comparison with the control soil, there was no significant effect of tuff mulching on CaCO3, pH, Mg, and OM of the soil.
Effect of tuff mulching on leaves minerals
The content of nitrogen, phosphorus, and sodium inside the leaves was significantly increased (p ≤ 0.01) by 27%, 60%, and 126%, respectively, for the tree grown in mulched soil (Fig. 8). In contrast, insignificant effects of soil mulching on leaves calcium, potassium, and magnesium content were observed as compared to the control treatment. Calcium and magnesium content in leaves grown in mulched soil is higher than that in leaves grown in silty clay soil by 2% and 18%, respectively, but this increase is statistically insignificant.