Mowing does not redress the negative effect of nutrient addition on alpha and beta diversity in a temperate grassland

Species loss due to an increasing number of added nutrients has been explained by both light competition through biomass increase and by niche dimension reduction as a result of species specific limiting soil resources trade-offs. Disturbances, by reducing community biomass, species dominance and increasing light availability, may counteract above ground nutrient effects. However, it is unknown if diversity loss at local or spatial scales generated by increasing number of added nutrients can be redressed with canopy disturbance.

We evaluated if local (alpha) and spatial scale (beta) diversity loss generated by the number of added nutrients can be reverted by disturbances in Flooding Pampa grasslands, Argentina. In a 4-yr replicated field experiment, we added soil resources combining nitrogen, phosphorus and potassium to obtain 0, 1, 2 or 3 nutrients and manipulated the regime of canopy disturbance by seasonal mowing and biomass removal.

We found that the increasing number of added nutrients strongly reduced local and spatial plant diversity, despite biomass and light changes generated by mowing. In mown plots, nutrient driven local diversity loss was intensified along time, thus increasing species dominance. While mowing did not affect dominant species loss, increasing number of added nutrients promoted rare species loss and reduced spatial dissimilarity. Furthermore, mowing increased local and spatial diversity regardless light or biomass effects, suggesting alternative pathway effects for disturbance.

Synthesis: Our results demonstrated that even when disturbance generated a positive effect on local and spatial diversity, it did not completely counteract the negative effect of number of added nutrients. Thus, the relative importance of above and belowground resource competition may change when chronic disturbances alter community dominance. Under low light availability, above-ground competition may drive species richness loss but when disturbance reduces light limitation, the increasing number of added nutrients may reduce niche dimensionality and thus species coexistence. In sum, faced with the need to manage eutrophized grasslands, our study showed that disturbance may not completely mitigate the negative effect of multiple nutrient inputs on local and spatial grassland diversi

Methods
In 2012 we established a long-term experiment of nutrient addition in six fenced areas excluded from grazing and incorporated mowing treatment in 2014 because canopy disturbances were absent. Our experiment consisted of a split-plot design, replicated in six blocks, with the addition of limiting soil resources in the main plot (5 x 5 m2) and mowing treatments as frequent canopy disturbance in the sub-plots (2,5 x 2,5 m2; Supplementary Fig. 1S). The nutrient addition experiment is part of the Nutrient Network (https://nutnet.org/https://nutnet.org/ https://nutnet.org/https://nutnet.org/www.nutnet.org), however mowing as treatment is exclusive of our site. Blocks consisted of six 20 x 25 m2 enclosures separated by ~100m from each other and were established in 2004, as part of a long-term experiment of cattle exclusion (Longo et al., 2013). A factorial arrangement of nitrogen [N], phosphorous [P] and potassium [K + micronutrients, hereinafter K] were randomly assigned to each of eight 25 m2 main plots to represent the minimum number of potentially limiting elemental nutrients added (Harpole & Tilman, 2007). In each block, nutrient addition followed a scheme of increasing number of limiting resources (hereafter, number of added nutrients), with zero nutrients added (1 plot/block), one nutrient added (N, P, or K; 3 plots/block), two nutrients added (NP, NK or PK; three plots/block), and three nutrients added (NPK; 1 plot/block). Each nutrient was added at a rate of 10g m2 year1 to ensure a substantial increase in nutrient availability and remove potential limitations (Fay et al., 2015). Doses were split and equally applied three times a year (1/3 in early spring, 1/3 in early summer and 1/3 in early autumn). The nutrients were applied in commercially available granules of urea (N), triple superphosphate (P) and potassium sulphate (K) (Borer et al., 2014). From the six blocks, three blocks contained the entire nutrient range (0-1-2-3) and the other three only contained the extreme values (0 and 3). We incorporated all blocks to increase statistical power. Mowing treatments involved mechanical slash and removal of aerial biomass to a height < 5 cm in one sub-plot. Thus, each main plot was divided in 4 subplots (2.5 x 2.5 m); mown subplot was opposed to the “intact” subplot to ensure less contact and thus more independence between mowing treatments (Fig. 1S). As for nutrients, mowing was applied three times a year at the beginning of spring, summer, and autumn [i.e. September, December and April, respectively].

Data collection

Every year, between 2014 and 2017, we measured vegetation in late spring (i.e. November) and late summer (i.e. March) in order to assess the effect of nutrient addition and mowing on diversity. Plant percentage cover in the permanent 1x1 m2 was visually estimated to the nearest 5% using a modified Daubenmire method (Daubenmire1959). In all analyses we used the maximum cover value recorded for each species in late spring or summer to accurately represent species’ performance in the entire year (Perelman et al., 2001).

            We measured peak plant biomass and light penetration simultaneously with plant cover measurements (i.e. November-March). Live and standing dead plant aerial biomass was harvested annually at peak biomass (i.e. early March; Sala, Deregibus, Schlichter, & Alippe, 1981), within two frames of 0.2 x 0.5 m2, randomly located in mown and intact subplots, avoiding the area used for plant cover estimation. Samples were dried at 60° C for 72 h. and weighed to the nearest 0,1 g. Photosynthetically active radiation (PAR, μmol m2 s1) was measured above and below vegetation at noon (11:00 and 14:00) on sunny days using a 1m-long ceptometer (Cavadevices, Buenos Aires, Argentina). Proportion of PAR reaching the soil (pPAR) was calculated as the ratio of below/above PAR. We used the minimum value from two readings to account for of light limitation.