Effects Of Soil Biota On Plant Responses To Soil Heterogeneity And N:P Stoichiometry

Soil biota(soil microorganisms and soil animals)may affect plant growth through enhanced plant nutrient responses.While plants are commonly associated with diverse communities of soil microbes,complementary roles of different microbial communities that may stimulate synergistic effects on plant growth are not adequately tested.Meanwhile,although many studies have examined effects of soil heterogeneity and nitrogen(N):phosphorous(P)stoichiometry on plant growth,few have tested how soil biota affects plant responses to soil heterogeneity and N:P stoichiometry.Such knowledge about roles of soil biota will promote our current understanding of the mechanisms that drive ecosystem productivity and multi-functionality.The aim of the study,therefore,is to examine the roles of soil biota in plant responses to soil heterogeneity and N:P stoichiometry,via four controlled greenhouse experiments with two well-studied,typical clonal plants Leymus chinensis and Solidago canadensis.Firstly,plants of S.canadensis were grown in the background soil with fresh soil inoculum collected from two different regions(north and south)of China,a mixture of the two types of fresh soil inocula and sterilized soil inoculum(control)under low or high nutrient availability and under low or high water supply.Biomass of S.canadensis was generally higher when the plant grew in the soil inoculated with a mixture of soil microbes from the south and the north of China than when it grew in the soil inoculated with soil microbes from only the north or only the south or the sterilized control(F=34.4,P<0.001),and such effects of soil microbes were stronger under high than under low nutrient availability(F=4.3,P=0.006)and under high than under low water supply(F=7.2,P<0.001).These results suggest that interactions of different soil microbial communities can result in a synergistic effect on plant growth,and such a synergistic effect depends on environmental conditions.Secondly,plants of L.chinensis were grown in either heterogeneous soils consisting of low-and high-nutrient soil patches or a homogeneous soil consisting of a uniform mixture of the same low-and high-nutrient soils.The homogeneous and heterogeneous soils were manipulated in three different substrates and inoculated with either live or sterilized soil biota.In the presence of soil biota L.chinensis produced more biomass in the homogeneous soil than in the heterogeneous soils,but in the absence of soil biota it produced less(F=11.0-21.2,all P<0.001).In the heterogeneous soils consisting of field soil(with low nutrients)and potting soil(with high nutrients),L.chinensis produced more biomass in the high-nutrient patches than in the low-nutrient patches.In contrast,in the heterogeneous soils consisting of field soil(with low nutrients)and a mixture of quartz and ceramsite(with high nutrients),L.chinensis produced more biomass in the low-nutrient patches than in the high-nutrient patches(F=31.3,P<0.001).Therefore,both soil biota and soil substrates can influence plant responses to soil nutrient heterogeneity.Thirdly,plants of L.chinensis were grown in two heterogeneous conditions consisting of patches with or without ¹⁵N-labelled litter and differing in patch size(with large and small patches,respectively)and a homogeneous condition with an even distribution of the ¹⁵N-labelled litter,crossed with two earthworm treatments(with or without earthworms,Eisenia fetida).Compared to the homogeneous treatment,the heterogeneous distribution of litter in large patches greatly increased biomass of L.chinensis without earthworms but decreased it with earthworms(F=15.6,P<0.001).Biomass of L.chinensis was significantly greater in patches with than without litter,showing foraging responses.However,the foraging response of belowground mass was much stronger in the small than in the large patch treatment without earthworms,but was similar with earthworms,as indicated by the significant effect of patch scale×earthworms×patch quality(F=5.2,P=0.029).These results suggest that the presence of earthworms can alter the effect of soil heterogeneity on plant performance and that such an effect of earthworms varies with patch scales.Finally,plants of S.canadensis were grown at three N:P ratios[low(1.7),intermediate(15)and high(135)]under low or high nutrient levels and with or without soil biota.Increasing the soil N:P ratio and the nutrient level significantly increased the growth of S.canadensis in the presence of soil biota.S.canadensis also responded positively to increased nutrient levels at the low N:P ratio in the absence of soil biota,but this response was eliminated by the presence of soil biota.The positive interaction of the plant and soil biota stimulated the accumulation of N and P in S.canadensis.Thus,soil biota can mediate N:P stoichiometric effects on plant growth.In conclusion,soil biota can influence plant responses to soil heterogeneity and N:P stoichiometry,likely via movement(homogenization)of nutrients between different patches,compensatory supply of the limiting soil nutrient,and niche differentiation in nutrient use between plants and soil biota.Such effects of soil biota may further influence community structure and ecosystem functioning.