62. Grassland plots with varying number of grass species were cultivated for 10 years. At the end of the experiment, total plant cover was measured. Soil nitrogen was also measured to assess its utilization by plants. The relationships are shown in the following plots.
    
    Which one of the following inferences can be drawn from the above experiment?
    (1) Grasses in plots with lower species richness enriched soil nitrogen, thereby increasing the plant cover.
    (2) Plots with greater species richness showed greater stability and more efficient soil nitrogen utilization.
    (3) Plots with greater species richness utilized nitrogen more efficiently, but would not show
    increased net primary production.
    (4) No correlation can be drawn between species richness, community productivity and nitrogen
    utilization.

The correct inference from the described grassland experiment is:

(2) Plots with greater species richness showed greater stability and more efficient soil nitrogen utilization.

Supporting evidence:

• Higher species richness in grassland plots is associated with more efficient soil nitrogen utilization. Studies have shown that as the number of plant species increases, the community more effectively partitions and utilizes available soil nitrogen, leading to lower residual soil nitrate levels.

• Greater species richness is linked to increased community productivity and stability. Experimental and meta-analytic evidence indicates that diverse grassland communities tend to have higher total plant cover and are more resistant to environmental fluctuations, reflecting greater stability and productivity.

• There is a positive relationship between species richness, community productivity, and nitrogen utilization. Plots with more species not only use nitrogen more efficiently but also tend to have higher biomass or plant cover, supporting the link between diversity and ecosystem function.

The other options are not supported:

• Option (1) is incorrect because plots with lower species richness do not enrich soil nitrogen; rather, higher diversity leads to more efficient nitrogen use.

• Option (3) is incorrect because increased species richness is associated with both more efficient nitrogen utilization and increased net primary production.

• Option (4) is incorrect as multiple studies demonstrate clear correlations between species richness, productivity, and nitrogen utilization.

“Higher diversity was associated with lower soil nitrate, while percentage vegetative cover and the presence of idiosyncratic species best predicted light extinction. The relationship between diversity, and plant biomass (measured as vegetative cover) and plant number was positive in the first year of the study.”

“We hypothesized that partitioning of different sources of soil nitrogen among four plant functional groups should increase with diversity.”

Thus, option (2) best summarizes the experimental findings and current scientific understanding.