The injectivity of Frobenius acting on cohomology and local cohomology modules

LetR be a two-dimensional normal graded ring over a field of characteristicp>0. We want to describe the tight closure of (O) in the local cohomology moduleH _R+ ² (R) using the graded module structure ofH _R+ ² (R). For this purpose we explore the condition that the Frobenius mapF: H _R+ ² (R)]_n→H _R+ ² (R)]_pninduced on graded pieces ofH _R+ ² (R) is injective. This problem is treated geometrically as follows: There exists an ample fractional divisorD onX=Proj (R) such thatR=R (X, D)= ⊕ _n≥0H⁰(X O _X (n D)). Then the above map is identified with the induced Frobenius on the cohomology groups Our interest is the casen<0, and in this case, a generalization of Tango's method for integral divisors enables us to show thatF _n is injective ifp is greater than a certain bound given explicitly byX andnD. This result is useful to studyF-rationality ofR. The notion ofF-rational rings in characteristicp>0 is defined via tight closure and is expected to characterize rational singularities. We ask if a modulop reduction of a rational signularity in characteristic 0 isF-rational forp≫0. Our result answers to this question affirmatively and also sheds light to behavior ofF-rationality in smallp.