The enzymatic latch hypothesis proposes that oxygen (O 2 ) limitation promotes wetland carbon (C) storage by indirectly decreasing the activities of hydrolytic enzymes that decompose organic matter. Humid tropical forest soils are often characterized by low and fluctuating redox conditions and harbor a large pool of organic matter, yet they also have the fastest decomposition rates globally. We tested the enzymatic latch hypothesis across a soil O 2 gradient in the Luquillo Experimental Forest, Puerto Rico, USA. Enzyme activities expressed on a soil mass basis did not systematically decline across a landscape O 2 gradient, nor did phenolics accumulate, the proposed mechanism of the enzymatic latch. Normalizing enzyme activities by C concentrations did suggest a decline in several enzymes as mean soil O 2 decreased. However, relationships between hydrolytic enzymes and reducing conditions were scale-dependent: enzymes displayed neutral to strongly positive relationships with reducing conditions and phenolics when comparing samples within sites, and enzyme activities in 18-d anaerobic incubations generally exceeded those in aerobic soils despite a fourfold increase in phenolics. In summary, although O 2 availability and the activities of some enzymes appeared to be related at landscape scales after accounting for differences in organic matter, reducing conditions and phenolic compounds did not appear to constrain soil hydrolytic enzyme activity at the scale of soil microsites, challenging the enzymatic latch hypothesis. Hydrolytic enzymes can be resilient to periodic anaerobiosis and may actually stimulate O 2 consumption at the microsite scale. We suggest a critical re-examination of mechanisms and the scale dependence of couplings between O 2 and decomposition in terrestrial soils.

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We built a collector to filter interplanetary dust particles (IDPs) larger than 5 lm from the clean air at the Amundsen Scott South Pole station. Our sampling strategy used long duration, continuous dry filtering of near-surface air in place of short duration, high-speed impact collection on flags flown in the stratosphere. We filtered~10 7 m 3 of clean Antarctic air through 20 cm diameter, 3 µm filters coupled to a suction blower of modest power consumption (5-6 kW). Our collector ran continuously for 2 years and yielded 41 filters for analyses. Based on stratospheric concentrations, we predicted that each month's collection would provide 300-900 IDPs for analysis. We identified 19 extraterrestrial (ET) particles on the 66 cm 2 of filter examined, which represented~0.5% of the exposed filter surfaces. The 11 ET particles larger than 5 µm yield about a fifth of the expected flux based on >5 µm stratospheric ET particle flux. Of the 19 ET particles identified, four were chondritic porous IDPs, seven were FeNiS beads, two were FeNi grains, and six were chondritic material with FeNiS components. Most were <10 µm in diameter and none were cluster particles. Additionally, a carbon-rich candidate particle was found to have a small 15 N isotopic enrichment, supporting an ET origin. Many other candidate grains, including chondritic glasses and C-rich particles with Mg and Si and FeS grains, require further analysis to determine if they are ET. The vast majority of exposed filter surfaces remain to be examined.

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