Human societies depend on an Earth System that operates within a constrained range of nutrient 68 availability, yet the recent trajectory of terrestrial nitrogen (N) availability is uncertain. 69 Examining patterns of foliar N concentrations ([N]) and isotope ratios (δ 15 N) from more than 42,000 samples acquired over years, here we show that foliar [N] declined by 8% and foliar δ 15 N declined by 0.8 -1.9 ‰. Examining patterns across different climate spaces, foliar δ 15 N declined across the entire range of MAT and MAP tested. These results suggest declines in N supply relative to plant demand at the global scale. In all, there are now multiple lines of evidence of declining N availability in many unfertilized terrestrial ecosystems, including declines in δ 15 N of tree rings and leaves from herbarium samples over the past 75-150 years. 76These patterns are consistent with the proposed consequences of elevated atmospheric CO 2 and longer growing seasons. These declines will limit future terrestrial C uptake and increase nutritional stress for herbivores. 235 much. Preventing these declines in N availability further emphasizes the need to reduce 236 anthropogenic CO 2 emissions.Data and code availability. The datasets generated during and/or analysed during the current study are available in the Dryad repository [link to be generated upon acceptance]. All code used for statistical analyses and figure generation are available on Dryad (XXX).

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Guano from breeding seabirds provides a large external source of nutrients to the soils of breeding colonies. However, little is known of guano P retention relative to N or the relative importance of guano and soil parent material as P sources. Soil profile N and P inventories (0–0.60 m, n = 4; 0–0.36 m, n = 1) and guano N and P concentrations were measured at a Westland petrel colony, and the parent material contributions of P were calculated using Ca, Al, Fe, Ti, and Zr as reference elements. Median inventories (0–0.60 m) were 1.49 kg N/m2 and 332 × 10–3 kg P/m2, the N result being similar to that from a seabird colony on peat soil where N retention was very low. Calculated parent material contributions were smallest (32–66% of soil P) when based on Ca and largest (47–102% of soil P) when based on Zr. Contributions were similar for Al, Fe, and Ti; Al (41–87% soil P) was selected for subsequent calculations. Regardless of the reference element, parent material therefore contributed a large part of soil P. Phosphorus in excess of parent material supply (Pexcess) was significantly correlated with soil C, implying that guano P is held primarily in organic form. The median soil N : P molar ratios were 9.6 : 1 based on total P and 11.2 : 1 based on Pexcess, compared with ratios for Westland petrel guano of 4.1 : 1 (when birds were consuming fisheries waste) and 16.4 : 1 (when fisheries waste was replaced by fish). The similarity between soil and guano N : P ratios implies that both N and P are lost from soil at similar rates, although volatilisation of N would enrich soil drainage water in P. Calculations using guano deposition rates from the literature yielded P residence times of 4–15 years (Pexcess) and 11–41 years (total P), consistent with a highly dynamic soil system.

The 3 Otago coastal water masses (Neritic water, subtropical Southland Current water, and Subantarctic Surface Water) defined in the literature were identified on 1 cruise in early summer and 1 in mid winter. Winter results showed that Neritic water is derived from the Clutha River. During winter the Southland Current was characterised by temperatures between 9.0 and 10.6°C and a salinity range of 34.4 to 34.9 × 10-3. During summer a thermocline and pycnocline were observed at 30-50 m depth sloping down toward the coast. Low-salinity Neritic water was less apparent than in winter. Summer nutrient levels were depleted over all of the continental shelf, relative to winter. This is probably caused by the enclosure of coastal surface waters inshore of the pycnocline. Reactive Si was conservative in winter regardless of water mass. Reactive P was approximately constant in Neritic water and Southland Current water. In summer, neither nutrient was conservative.