The Effects of Ecological Restoration on Nitrogen Partitioning in Wetland Ecosystems

<p dir="ltr">Nitrogen (N) is essential for plant growth, but excessive inputs from fertilisers, livestock waste, and non-native N-fixing plants can overwhelm ecosystems. Wetlands, often referred to as “nature’s kidneys”, play a key role in buffering excess N. However, despite their importance, over 90% of New Zealand’s wetlands have been lost or degraded, and our understanding of how wetland condition influences N pools and fluxes remains limited. Ecological restoration is the process of repairing human-altered ecosystems to resemble their approximate natural structure and function. Research on N dynamics in restored wetlands, particularly those converted from agroecosystems, is sparse. Wetland swamp forests, despite having the highest N retention capacity among wetland types, also remain largely understudied. My thesis aimed to address these knowledge gaps by examining how ecological restoration affects N dynamics (pools and fluxes) across three wetland states: unrestored wetlands (actively disturbed by agriculture), restored wetlands (undergoing active restoration following agriculture), and remnant wetlands (mature swamp forest). I sampled soils, leaves, roots, wood, and bark from 19 wetlands across the Wairarapa region of New Zealand. Samples were analysed for percent N (%N) and δ¹⁵N values, with soils additionally analysed for mineral and microbial N. I hypothesised that (1) plant tissue %N would be highest in remnant wetlands, due to greater plant functional diversity and biomass; (2) soils in unrestored wetlands would have higher total N and mineral N content, but lower microbial N, due to different N source inputs, reduced plant biomass, and lower amounts of soil organic matter storing less N; (3) plant tissue δ¹⁵N values would be lowest in remnant wetlands, followed by restored and unrestored wetlands, mirroring dominant N source inputs; and (4) soil δ¹⁵N values would show the same pattern, with remnant wetlands having the lowest δ¹⁵N values due to greater soil organic matter and microbial diversity, which reduces ¹⁴N loss. </p><p dir="ltr">I found total ecosystem N ranged from 504 g m⁻² (± 82.2 SE) in unrestored wetlands to 654 g m⁻² (± 156 SE) in restored wetlands and 906 g m⁻² (± 118 SE) in remnant wetlands. However, these differences were not statistically significant due to high variability among wetlands of each state. When total ecosystem N was partitioned, linear models showed remnant wetlands stored significantly more N in above-ground biomass (herb and canopy foliage) than restored and unrestored wetlands. Surprisingly, soil percent N and mineral N (nitrate (NO₃⁻) and ammonium (NH4+)) did not differ significantly among wetland states, but when bulk density of soil was accounted for, the soil mineral fraction NO₃⁻ was greatest in unrestored wetlands. Microbial N differed significantly by wetland state, with remnant wetlands holding more N in microbial biomass than unrestored and restored wetlands. Remnant wetlands had the lowest δ¹⁵N values in soil, coarse roots, fine roots, and canopy foliar N pools. Herb layer δ¹⁵N did not differ between remnant and restored wetlands but was significantly higher in unrestored wetlands. Soil and fine root δ¹⁵N values were similar between unrestored and restored wetlands. Overall, my findings highlight the complex responses of wetland N pools and fluxes to ecosystem state. Restoration increases N uptake and long-term N storage, but restored wetlands still hold less N than remnant wetlands, possibly indicating ongoing capacity for further uptake as restored wetlands mature. These results underscore the critical importance of protecting and restoring wetlands to remove excess N from landscapes, safeguarding freshwater and human health and enhancing landscape resilience.</p>